Add a null filter to re-enable frame count

[tvercaut]

This aims at addressing https://github.com/imageio/imageio-ffmpeg/issues/99

[almarklein]

CI fails on one specific test: https://github.com/imageio/imageio-ffmpeg/blob/9ab981a8d09138fb1b3b814cb4287b705f258ec5/tests/test_io.py#L40-L43

The result is 14.0, so it fails. I confirmed (in #108) that without the change in this PR, this failure does not happen. So apparently ffmpeg produces a slightly different value, depending on these output args.

Nonetheless, I think its safe to just update theupper bound to <= 14.0 in this test.

[kkoomen]

I can confirm this fix works for M1/M2 macbooks as well.

[almarklein]

@tvercaut could you update the test I referenced in my earlier comment? That should make the tests pass, so we can merge this 🚀

[kkoomen]

@almarklein can't you just update the tests? Looks like OP isn't that active.

[tvercaut]

I just pushed the requested test relaxation change for the PR.

[kkoomen]

@almarklein can you publish a new version pypi version?

[almarklein]

done!