(fix): checking None membership in __getitem__

[ilan-gold]

I ran into this issue when trying to index with a tuple that had a numpy array in it:

import finch
import numpy as np

tensor = finch.Tensor(np.array([[1, 2], [3, 4]]))
tensor[finch.Tensor(np.array([False, True]))]

Previously, None in key would trigger None==k for k in key (see the link in the comment) which is not really compatible with numpy arrays. Maybe key shouldn't be a numpy array though but couldn't tell? Happy to update.

I don't see any indexing tests that have a numpy array in https://github.com/finch-tensor/finch-tensor-python/blob/main/tests/test_indexing.py though...but also array-api compat would say boolean indexes are good https://data-apis.org/array-api/latest/API_specification/indexing.html#boolean-array-indexing

[hameerabbasi]

A regression test would be appreciated here!