fix:#95 with null-safety

[domliang]

fix #95 with null-safety

[sbis04]

Hi @domliang the real reason that I found while testing, the videos where this error is occurring don't contain proper Rotation information (so it is set to 0). You can also find some discussion about it here which causes various other video parsing issues (and it mostly affects the iOS device).

There's also a PR related to this which might fix this video parsing issue, here. But unfortunately, it isn't merged yet (and, it's a 2 year old PR 😞 ).

For example, I got an error like this using the video provided by @Vitaly-V:

TX and TY are 0. Rotation: 0. Natural width,height: 640.000000, 360.000000

This is the reason video_thumbnail package can't generate the thumbnails for that part of the video where it lacks the Rotation information (or, it is set to 0 by default).

For now, it's better to use this fix by reusing the previous correctly generated thumbnail where it returns null.

[sbis04]

Thanks for this PR 🎉

LGTM