fix(eslint): Add no-eq-null rule

.eslintrc.js

         'no-else-return': 2,
         'no-empty-function': 2,
         'no-empty-pattern': 2,
+        'no-eq-null': 2,
         'no-eval': 2,
         'no-extend-native': 2,
         'no-extra-label': 2,

modules/RTC/TraceablePeerConnection.js

  * Returns a string representation of a SessionDescription object.
  */
 const dumpSDP = function(description) {
-    if (typeof description === 'undefined' || description == null) {
+    if (typeof description === 'undefined' || description === null) {
         return '';
     }
 

modules/xmpp/SDP.js

                         // but msid stands for the stream id, makes no sense ?
                         msid = localTrack.getTrackId();
                     }
-                    if(msid != null) {
+                    if(msid !== null) {
                         msid = SDPUtil.filter_special_chars(msid);
                         elem.c('parameter');
                         elem.attrs({name: 'msid', value:msid});

modules/xmpp/xmpp.js

         }
         // Constant JIDs need some random part to be appended in order to be
         // able to join the MUC more than once.
-        if (this.authenticatedUser || cfgNickname != null) {
+        if (this.authenticatedUser || cfgNickname !== null) {
             mucNickname += '-' + RandomUtil.randomHexString(6);
         }