webrtchttp: Allow audio or video caps to be specified as None with WHEP

We were setting audio and video caps by default even when the user might have requested only video or audio. This would then result in a Could not reuse transceiver error from the webrtcbin.

Fix this by allowing the user to specify audio or video caps as None. This allows us to maintain the earlier behaviour for backward compatibility while allowing the user to not request audio or video as need be.

Fixes #484 (closed).