Fix Firefox compatibility by handling null ICE candidates

[YPandas]

Issue #, if available:

• Firefox sends a null candidate to signal end of ICE gathering, which caused parsing errors when trying to process these candidates.

Description of changes:

• Add null checks before sending ICE candidates
• Prevents errors when ICE gathering completes and the candidate with field 'candidate' being null is sent

BEFORE

AFTER

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[unicornss]

let us test in diff browsers

The base branch was changed.