Exclude empty paths on ChunkDict creation

[ghidalgo3]

This was discovered while experimenting with GOES data, kudos to @t-mcneely for discovering this. We think there's a small bug in the filter expression during ChunkDict creation. Ultimately this caused a problem when reading/writing kerchunk stores of the data.

Here's a neat image rendering the chunk grid we were working with, note how the corners are empty:

• Tests added
• Tests passing
• Full type hint coverage
• Changes are documented in docs/releases.rst

[TomNicholas]

Thanks!

FYI I'm currently actually road-tripping and just occasionally looking at all the great PRs you are raising from my phone... Let me know if it would be useful to give more people push access.

[ghidalgo3]

Enjoy your road trip! If you could give @TomAugspurger access, I trust him to review my changes, but please don't feel like you need to keep up with PRs on the road. I can always work off a fork.