webrtc: fix Safari addIceCandidate crash

The Safari browser does not adhere to the specifications outlined at: https://developer.mozilla.org/en-US/docs/Web/API/RTCPeerConnection/addIceCandidate and crashes when the candidate is an empty string. This PR resolves the issue and is compatible with other browsers (tested on Chrome).

Passing "null" as candidate is still compliant with the standard:

If the candidate parameter is missing or a value of null is given when calling addIceCandidate(), the added ICE candidate is an "end-of-candidates" indicator. The same is the case if the value of the specified object's candidate is either missing or an empty string (""), it signals that all remote candidates have been delivered.

I'm aware of #439 (closed) (patching of webrtc-adapter) but by some reason it doesn't work for me...