fix(TPCUtils): undefined is not an object (evaluating 'this.tpcUtils.replaceTrack(e,t).then')

By looking at the replaceTrack usages it's possible for it to be called with both new and old tracks null/undefined.

modules/RTC/TPCUtils.js

@@ -365,6 +365,8 @@ export class TPCUtils {
                     this.pc.localSSRCs.set(newTrack.rtcId, ssrc);
                 });
         }
+
+        return Promise.resolve();
     }
 
     /**