Don't use empty `asserts`

[timtreis]

I remember seeing quite a few GitHub issues that are opened because users run into assert statements that just fail but don't educate the user why they were caused. I think we should have an internal policity to not have that and instead raise proper errors. That'd also make triaging issues for us easier.

Will continue to link related issues below:

• render_shapes fails when +1 shapes match single table.obs spatialdata-plot#475