fix: support is_indexed types in ak.fill_none

[agoose77]

Fixes #2107

[codecov]

Codecov Report

Merging #2111 (ff67e0f) into main (6efa287) will not change coverage.
The diff coverage is 100.00%.

Additional details and impacted files

Impacted Files	Coverage Δ
src/awkward/operations/ak_fill_none.py	95.00% <100.00%> (ø)

[jpivarski]

Old logic: "if union or record, continue recursing (by returning None); if option-type, do fill_none; otherwise, stop here (by returning layout)."

New logic: "if option-type, do fill_none; if union, record, or indexed (not IndexedOptionType because this is after the check for option-type), continue recursing; otherwise, stop here."

That makes sense. It could be an IndexedArray of RecordArray (as in your test). In fact, that's pretty common, since any carry of a RecordArray results in an IndexedArray of RecordArray.