fix: visit all layout nodes with ak.fill_none(..., axis=None)

[agoose77]

Fixes #1823

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📚 The documentation for this PR will be available at https://awkward-array.readthedocs.io/en/agoose77-fix-fill-none-axis-none/ once Read the Docs has finished building 🔨

[codecov]

Codecov Report

Merging #1830 (37ff8be) into main (2bdd114) will increase coverage by 0.00%.
The diff coverage is 100.00%.

Additional details and impacted files

Impacted Files	Coverage Δ
src/awkward/operations/ak_fill_none.py	94.87% <100.00%> (+5.12%)	⬆️
src/awkward/contents/indexedarray.py	79.46% <0.00%> (-0.20%)	⬇️
src/awkward/_util.py	82.36% <0.00%> (-0.18%)	⬇️
src/awkward/contents/indexedoptionarray.py	88.90% <0.00%> (-0.16%)	⬇️
src/awkward/_connect/avro.py	87.17% <0.00%> (-0.14%)	⬇️
src/awkward/types/_awkward_datashape_parser.py	47.72% <0.00%> (-0.01%)	⬇️
src/awkward/contents/recordarray.py	83.94% <0.00%> (+0.03%)	⬆️
src/awkward/contents/content.py	77.31% <0.00%> (+0.10%)	⬆️

[jpivarski]

Nice test and nice fix!