fix(eslint): Add no-empty-function rule

.eslintrc.js

         'no-case-declarations': 2,
         'no-div-regex': 0,
         'no-else-return': 2,
+        'no-empty-function': 2,
         'no-empty-pattern': 2,
         'no-eval': 2,
         'no-extend-native': 2,

JitsiConference.js

         }
         try {
             this._addLocalStream(
-                localTrack.getOriginalStream(), function() {}, function() {},
-                ssrcInfo, true /* don't modify SSRCs */);
+                localTrack.getOriginalStream(),
+                () => {
+                    // There is nothing to be done after the stream is added.
+                },
+                () => {
+                    // TODO: Maybe report the error to the user?
+                },
+                ssrcInfo,
+                true /* don't modify SSRCs */);
         } catch(e) {
             GlobalOnErrorHandler.callErrorHandler(e);
             logger.error(e);

JitsiParticipant.js

         return this._isMediaTypeMuted(MediaType.VIDEO);
     }
 
-    /**
-     * @returns {???} The latest statistics reported by this participant (i.e.
-     * info used to populate the GSM bars)
-     * TODO: do we expose this or handle it internally?
-     */
-    getLatestStats() {
-    }
-
     /**
      * @returns {String} The role of this participant.
      */
         return this._role;
     }
 
-    /**
-     * @returns {Boolean} Whether this participant is the conference focus (i.e.
-     * jicofo).
-     */
-    isFocus() {
-    }
-
-    /**
-     * @returns {Boolean} Whether this participant is a conference recorder
-     * (i.e. jirecon).
-     */
-    isRecorder() {
-    }
-
-    /**
-     * @returns {Boolean} Whether this participant is a SIP gateway (i.e.
-     * jigasi).
-     */
-    isSipGateway() {
-    }
-
-    /**
-     * @returns {Boolean} Whether this participant is currently sharing their
-     * screen.
-     */
-    isScreenSharing() {
-    }
-
-    /**
-     * @returns {String} The user agent of this participant (i.e. browser
-     * userAgent string).
-     */
-    getUserAgent() {
-    }
-
-    /**
-     * Kicks the participant from the conference (requires certain privileges).
-     */
-    kick() {
-    }
-
-    /**
-     * Asks this participant to mute themselves.
-     */
-    askToMute() {
-    }
-
     supportsDTMF() {
         return this._supportsDTMF;
     }

modules/RTC/JitsiRemoteTrack.js

 function JitsiRemoteTrack(rtc, conference, ownerEndpointId, stream, track,
                           mediaType, videoType, ssrc, muted) {
     JitsiTrack.call(
-        this, conference, stream, track, function() {}, mediaType, videoType, ssrc);
+        this,
+        conference,
+        stream,
+        track,
+        () => {
+            // Nothing to do if the track is inactive.
+        },
+        mediaType,
+        videoType,
+        ssrc);
     this.rtc = rtc;
     this.ownerEndpointId = ownerEndpointId;
     this.muted = muted;

modules/RTC/JitsiTrack.js

  */
 // eslint-disable-next-line no-unused-vars
 JitsiTrack.prototype._attachTTFMTracker = function(container) {
+    // Should be defined by the classes that are extending JitsiTrack
 };
 
 /**
  * screen capture as opposed to a camera.
  */
 JitsiTrack.prototype.isScreenSharing = function() {
+    // FIXME: Should be fixed or removed.
 };
 
 /**

modules/RTC/RTC.js

         }
     }
 
-    dispose() { }
-
     setAudioLevel(resource, audioLevel) {
         if(!resource) {
             return;

modules/RTC/TraceablePeerConnection.js

     if (RTCBrowserType.isFirefox()
             || RTCBrowserType.isTemasysPluginUsed()
             || RTCBrowserType.isReactNative()) {
-        // ignore for now...
         if(!errback) {
-            errback = function() {};
+            errback = function() {
+                // Making sure that getStats won't fail if error callback is
+                // not passed.
+            };
         }
         this.peerconnection.getStats(null, callback, errback);
     } else {

modules/statistics/AnalyticsAdapter.js

  * Interface for analytics handlers.
  */
 class AnalyticsAbstract {
-    sendEvent() {}
+    sendEvent() {} // eslint-disable-line no-empty-function
 }
 
 /**

modules/xmpp/JingleSession.js

      * {@link initialize}.
      * @protected
      */
-    doInitialize() {
-
-    }
+    doInitialize() { } // eslint-disable-line no-empty-function
 
     /**
      * Adds the ICE candidates found in the 'contents' array as remote
      * candidates?
      * Note: currently only used on transport-info
      */
-    // eslint-disable-next-line no-unused-vars
-    addIceCandidates(contents) {
-
-    }
+    // eslint-disable-next-line no-unused-vars, no-empty-function
+    addIceCandidates(contents) { }
 
     /**
      * Returns current state of this <tt>JingleSession</tt> instance.
      *
      * @param contents an array of Jingle 'content' elements.
      */
-    // eslint-disable-next-line no-unused-vars
-    addSources(contents) {
-
-    }
+    // eslint-disable-next-line no-unused-vars, no-empty-function
+    addSources(contents) { }
 
     /**
      * Handles a 'remove-source' event.
      *
      * @param contents an array of Jingle 'content' elements.
      */
-    // eslint-disable-next-line no-unused-vars
-    removeSources(contents) {
-
-    }
+    // eslint-disable-next-line no-unused-vars, no-empty-function
+    removeSources(contents) { }
 
     /**
      * Terminates this Jingle session by sending session-terminate
      * @param failure a callback called when either timeout occurs or ERROR
      * response is received.
      */
-    // eslint-disable-next-line no-unused-vars
-    terminate(reason, text, success, failure) {
-
-    }
+    // eslint-disable-next-line no-unused-vars, no-empty-function
+    terminate(reason, text, success, failure) { }
 
     /**
      * Handles an offer from the remote peer (prepares to accept a session).
      * error object with details(which is meant more to be printed to the logger
      * than analysed in the code, as the error is unrecoverable anyway)
      */
-    // eslint-disable-next-line no-unused-vars
-    acceptOffer(jingle, success, failure) {
-
-    }
+    // eslint-disable-next-line no-unused-vars, no-empty-function
+    acceptOffer(jingle, success, failure) { }
 }

modules/xmpp/RtxModifier.spec.js

     });
 
     describe('stripRtx', function() {
-        beforeEach(function() {
-        });
+        beforeEach(function() { }); // eslint-disable-line no-empty-function
         it('should strip all rtx streams from an sdp with rtx', function() {
             const sdpStr = transform.write(SampleSdpStrings.rtxVideoSdp);
             const newSdpStr = this.rtxModifier.stripRtx(sdpStr);