Prevent TransformMatchingTex from crashing when there is nothing to fade

[kjlubick]

Overview: What does this pull request change?

Fixes #2845 and adds tests.

I originally tried to make FadeTransformPieces not crash if it was given two Mobjects with no submobjects, but I couldn't quite get that to work. This is probably the less invasive of a change.

Reviewer Checklist

• [ ] The PR title is descriptive enough for the changelog, and the PR is labeled correctly
• [ ] If applicable: newly added non-private functions and classes have a docstring including a short summary and a PARAMETERS section
• [ ] If applicable: newly added functions and classes are tested

[kjlubick]

I also was not able to figure how to look at the test images, nor find docs on how to do that. Can someone give me a hint so I can make sure my tests look as I expect?

[kilacoda]

Thanks for your contribution! Please check out https://docs.manim.community/en/stable/contributing/testing.html#how-it-works for info on the tests.

[behackl]

Converting to draft for now, please un-draft or re-request a review / leave some comments if you'd like to discuss this further!

[kjlubick]

Sure thing. I hope to have some time tonight to implement your suggestion.