fix: don't preserve unexpected option for is_in

[agoose77]

PyArrow 14 started returning an option type, which follows from the new null_matching_behavior option.

@jpivarski I'm wondering about the _drop_option_preserving_form helper function that we use to erase options. Should we only check the bitmasks being erased in our test suite? i.e., at production, we don't assert that each entry is 0, but rather assume that the operation doesn't do this — the mask is just PyArrow adding a mask node?

[codecov]

Codecov Report

Merging #2792 (fb492c2) into main (2fbaa2c) will increase coverage by 0.00%.
The diff coverage is 100.00%.

Additional details and impacted files

Files	Coverage Δ
src/awkward/operations/str/__init__.py	100.00% <100.00%> (+0.88%)	⬆️
src/awkward/operations/str/akstr_index_in.py	97.14% <100.00%> (-0.08%)	⬇️
src/awkward/operations/str/akstr_is_in.py	97.14% <100.00%> (-0.08%)	⬇️

... and 1 file with indirect coverage changes

[jpivarski]

This is why Arrow's major version is increasing so rapidly: they change their API a lot!

Yes, this is good to merge, if it makes our string functions insensitive to this change in Arrow 14.