handle no audio

[ryanontheinside]

when no audio is extracted, return silence

[AustinMroz]

Thanks for for taking the time to open a pull request.

The not yet merged commit 9405e80 adds an alternative fix which, compared to the one here

• Doesn't create a -audio output file when no audio is extracted
• Still raises an error if that error is used on a different node that requires audio (Such as a VAEEncodeAudio for a StableAudio workflow).
• Immediately errors if the audio was loaded by a Load Audio node where audio is explicity desired.