Handle `NoneType` in `MirrorPath.from_s3_key()`

[TrevorBurgoyne]

Since MirrorPath.from_s3_key() uses str.removesuffix(), it breaks when a non-str is passed. Checking for type would be nifty to save headaches.

Error when calling MirrorPath.from_s3_key(None)

File "C:\Users\Trevor\py-rogue-detection\ddb_tracking\grd_structure.py", line 122, in from_json
 rogue_labels=MirrorPath.from_s3_key(json_dict["rogue_labels"]),
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "C:\Users\Trevor\miniconda3\envs\rogue-venv\Lib\site-packages\S3MP\mirror_path.py", line 50, in from_s3_key
 s3_key = s3_key.removesuffix('/')
          ^^^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'removesuffix'

[joshua-dean]

Would preferred behavior be raise a warning and default to "", or something else?
If there's not any sort of default behavior, it seems like this would just move the AttributeError upstream.

We could also set the behavior via the configuration.

[TrevorBurgoyne]

Raising a warning and returning None or "" seems good. In my particular use case returning None is what I ended up doing anyways