fix: null check on getLocalDescription Android

[renefloor]

Android is the only place where we don't expect local description to be null, which can lead to a crash on sdp.description.
Other parts (iOS and Flutter) already expect local description to be nullable.

Existing iOS code:

    if (peerConnection) {
      RTCSessionDescription* sdp = peerConnection.localDescription;
      if (nil == sdp) {
        result(nil);
      } else {
        NSString* type = [RTCSessionDescription stringForType:sdp.type];
        result(@{@"sdp" : sdp.sdp, @"type" : type});
      }
    } 

Existing Flutter code:

      final response =
          await WebRTC.invokeMethod('getLocalDescription', <String, dynamic>{
        'peerConnectionId': _peerConnectionId,
      });

      if (null == response) {
        return null;
      }
      String sdp = response['sdp'];
      String type = response['type'];
      return RTCSessionDescription(sdp, type);