Attempting to caption a video with no audio track results in a 500 when pulling from amara.org

[mfairchild365]

This should be a low priority because few videos should ever need captions when they don't have an audio track.

ffmpeg returns this error Stream map '0:a' matches no streams. because we map the audio output via $command .= ' -map 0:a'; //Map the audio to the output in https://github.com/unl/UNL_MediaHub/blob/4.0_template/src/UNL/MediaHub/Muxer.php#L102

It might be first checking the video has audio via mediainfo before adding $command .= ' -map 0:a'; //Map the audio to the output