Chore: fix python type hints

[mackenzie-grimes-noaa]

Correct some python type hint decorators which didn't match the actual runtime variables returned from functions in aws_utils.py and utils.py.

• For example, functions that return a Sequence, or sometimes None on error, were annotated to always return a Sequence.
• Or a function parameter which default to None was typed as non-null, so linters would complain if you omitted the parameter but the code would execute without issue.

[Geary-Layne]

Good cleanup. I didn't know about Optional or Generator typing.

[Geary-Layne]

This does not regard any code changes made as part of this pull request: I wonder if we have a team standard for when to raise an exception vs (maybe in addition too) logging.

[mackenzie-grimes-noaa]

Good cleanup. I didn't know about Optional or Generator typing.

I do like Optional, if only because it's a little less to type than Union[<my type>, None]. Code linters should interpret them the same. I think it's the closest Python has to what other languages call "Nullable"