[python] GIF generation fails on audio channel missing about loopygen

[Montspy]

Reproduces on problematic install with this command:

./docker.sh ffmpeg -loglevel info -y -ss 00:00:00 -to 00:00:02 -f lavfi -i anullsrc -f lavfi -i anullsrc  -filter_complex "amerge=inputs=2[a]" -map "[a]" -ac 2 test.mp3

Command output extract:

Input #0, lavfi, from 'anullsrc':

  Duration: N/A, start: 0.000000, bitrate: 705 kb/s
    Stream #0:0: Audio: pcm_u8, 44100 Hz, stereo, u8, 705 kb/s
Input #1, lavfi, from 'anullsrc':

  Duration: N/A, start: 0.000000, bitrate: 705 kb/s
    Stream #1:0: Audio: pcm_u8, 44100 Hz, stereo, u8, 705 kb/s
Filter amerge has an unconnected output

[Montspy]

Potential fix to be tested in commit 14d9783

[Montspy]

Could not get user to test the fix, but it has been working well on my machine.
Cherry-picked the fix into PR #47 (commit 26eb5b2)
Will close this issue once it reaches main branch