Guard against nil OnLocalCandidate handler
Guard against nil OnLocalCandidate handler

Currently the ICEGather will crash if trickle is enabled and
OnLocalCandidate hasn't been defined. This adds a simple nil check
to make sure we don't crash if the user doesn't set a OnLocalCandidate
callback

pion/webrtc,pion/webrtc,pion/webrtc,pion/webrtc