CastReceiver errors on first stats update when updated to new code

[theodab]

If we update the cast receiver to the current master, there's an error on the first stats update.
This happens when getting the video stream; this happens because activeStreams_.hasPeriod returns true if there's a text track, but StreamUtils.getVariantByStreamIds gets variants by the video and audio stream Ids. So if we have a text track in a period but no video or audio yet, the returned variant is null. Our code in getStats does not account for that possibility.

[joeyparrish]

Fix cherry-picked for v2.4.6.