fix(TPC): crash on FF accessing LD/RD

Firefox return null/undefined for localDescription/remoteDescription
objects if they have not been set yet, while Chrome does return empty
object instead. This commit makes the behaviour consistent by making
sure that at least empty object is returned for all browsers.

modules/RTC/TraceablePeerConnection.js

@@ -949,7 +949,7 @@ var getters = {
                     + '(local video mute hack)', desc);
         }
 
-        return desc;
+        return desc ? desc : {};
     },
     remoteDescription:  function() {
         var desc = this.peerconnection.remoteDescription;
@@ -961,7 +961,7 @@ var getters = {
             this.trace(
                 'getRemoteDescription::postTransform (Plan B)', dumpSDP(desc));
         }
-        return desc;
+        return desc ? desc : { };
     }
 };
 Object.keys(getters).forEach(function (prop) {