Fix Firefox compatibility by handling null ICE candidates

Author: Yuqi Huang

examples/answerer.js

@@ -97,7 +97,7 @@ class Answerer {
             // `candidate` will be the empty string if the event indicates that there are no further candidates
             // to come in this generation, or null if all ICE gathering on all transports is complete.
             // https://developer.mozilla.org/en-US/docs/Web/API/RTCPeerConnection/icecandidate_event
-            if (candidate) {
+            if (candidate && candidate.candidate) {
                 console.log(this._loggingPrefix, 'Generated ICE candidate for', this._remoteClientId || 'remote');
                 console.debug(this._loggingPrefix, 'ICE candidate:', candidate);
 
@@ -111,6 +111,10 @@ class Answerer {
                     }
                 }
             } else {
+                if(candidate && !candidate.candidate) {
+                    //firefox special case, candidate with null candidate field
+                    return;
+                }
                 console.log(this._loggingPrefix, 'All ICE candidates have been generated for', this._remoteClientId || 'remote');
 
                 // When trickle ICE is disabled, send the answer now that all the ICE candidates have been generated.