fix(dropbox-recording) Prevent start recording when no dropbox token

lang/main.json

         "muteParticipantsVideoButton": "Disable camera",
         "muteParticipantsVideoTitle": "Disable camera of this participant?",
         "muteParticipantsVideoBody": "You won't be able to turn the camera back on, but they can turn it back on at any time.",
+        "noDropboxToken": "No valid Dropbox token",
         "Ok": "OK",
         "password": "Password",
         "passwordLabel": "The meeting has been locked by a participant. Please enter the $t(lockRoomPassword) to join.",

react/features/recording/components/Recording/AbstractStartRecordingDialog.js

     getDropboxData,
     isEnabled as isDropboxEnabled
 } from '../../../dropbox';
+import { showErrorNotification } from '../../../notifications';
 import { toggleRequestingSubtitles } from '../../../subtitles';
 import { setSelectedRecordingService } from '../../actions';
 import { RECORDING_TYPES } from '../../constants';
         let appData;
         const attributes = {};
 
-        if (_isDropboxEnabled
-                && _token
-                && this.state.selectedRecordingService
-                    === RECORDING_TYPES.DROPBOX) {
-            appData = JSON.stringify({
-                'file_recording_metadata': {
-                    'upload_credentials': {
-                        'service_name': RECORDING_TYPES.DROPBOX,
-                        'token': _token
+        if (_isDropboxEnabled && this.state.selectedRecordingService === RECORDING_TYPES.DROPBOX) {
+            if (_token) {
+                appData = JSON.stringify({
+                    'file_recording_metadata': {
+                        'upload_credentials': {
+                            'service_name': RECORDING_TYPES.DROPBOX,
+                            'token': _token
+                        }
                     }
-                }
-            });
-            attributes.type = RECORDING_TYPES.DROPBOX;
+                });
+                attributes.type = RECORDING_TYPES.DROPBOX;
+            } else {
+                dispatch(showErrorNotification({
+                    titleKey: 'dialog.noDropboxToken'
+                }));
+
+                return;
+            }
         } else {
             appData = JSON.stringify({
                 'file_recording_metadata': {