The muxer might not have accepted the first sample, if it
is waiting for audio track.
This bug causes issue when
1. VideoSampleExporter gives first sample (timestamp = 0) to the muxer.
2. Muxer does not write it because its waiting for audio track.
3. The video pipleline has processed all the sample and they are ready
to be consumed.
4. VideoSampleExporter fetches the next available sample from encoder (which is still with timestamp = 0) but it changes its timestamp to last timestamp because VideoSampleExporter thinks it has muxed the sample at timestamp zero, but in reality it hasn't. This is because the flag `hasMuxedTimestampZero` is set when queueing the input, rather than actually muxing the input.
This scenario can happen when video is processed much faster than
the audio.
PiperOrigin-RevId: 675565603
acb8e71c6e