6.2.1 Won't Process File With Missing Audio Stream

**FastFlix Version:**
6.2.1

**Operating System:**
Debian Trixie

**Describe the bug**
When encoding files that do not have an audio stream encoding fails:
```
[matroska @ 0x55b514da0c00] Only audio, video, and subtitles are supported for Matroska.
[out#0/matroska @ 0x55b514da0ac0] Could not write header (incorrect codec parameters ?): Invalid argument
[vf#0:0 @ 0x55b514dc7700] Error sending frames to consumers: Invalid argument
[vf#0:0 @ 0x55b514dc7700] Task finished with error code: -22 (Invalid argument)
[vf#0:0 @ 0x55b514dc7700] Terminating thread with return code -22 (Invalid argument)
[out#0/matroska @ 0x55b514da0ac0] Nothing was written into output file, because at least one of its streams received no packets.
frame= 0 fps=0.0 q=0.0 Lsize= 0KiB time=N/A bitrate=N/A speed=N/A elapsed=0:00:00.12

encoded 0 frames
Conversion failed!
```

I normally use Fastflix to process Prores files that some of them have no audio and remux the audio stream into the MKV files when the encoding is complete.

Fastflix 6.0.1 worked fine with files without audio but upgrading to 6.2.1 results in this error. I've rolled back to 6.0.1 for the time being.