This change avoids a muxer deadlock when:
1. Sequence of items
2. First item has audio track that is shorter than video
3. Audio finishes, and muxer refuses to write more than 500ms of video
consecutively.
SequenceAssetLoader fails to progress to the second item. A muxer
deadlock is possible when the audio of the first item finishes,
audio end-of-stream is not propagated through AudioGraph, and muxer blocks
video, preventing SequenceAssetLoader to move to the next item in sequence.
By triggering silence generation early as soon as audio EOS is
encountered, we ensure SequenceAssetLoader can progress to the next item.
PiperOrigin-RevId: 636179966
a74076f691