Fix: check output, if output is nil then ignore.

[jinleileiking]

This will cause panic later.

https://github.com/livekit/ingress/blob/a7b821a5324e1964b4bed7f524470be31b90ca00/pkg/media/webrtc.go#L167

[biglittlebigben]

You are right that we should handle this returned error. I'm however not sure whether we should fail the whole ingress instead of skipping the filling output. In which case did you see the NewVideoOutput failing?

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