fix: support scalar strings in `fill_none` #1708

Job	Run time
Linux (3.12, -r requirements-test.txt)	2m 49s
Linux-ROOT-dask-awkward (3.8)	4m 52s
Linux (3.11, -r requirements-test.txt)	4m 9s
Linux (3.10, -r requirements-test.txt)	5m 9s
Linux (3.9, -r requirements-test.txt)	4m 47s
MacOS (3.12)	2m 2s
Linux (3.8, -r requirements-test.txt)	4m 57s
MacOS (3.11)	4m 22s
Linux (3.8, -r requirements-test-minimal.txt)	2m 36s
MacOS (3.10)	5m 11s
MacOS (3.9)	4m 57s
MacOS (3.8)	4m 54s
Windows (3.12, x64)	2m 20s
Windows (3.11, x64)	4m 50s
Windows (3.10, x64)	3m 25s
Windows (3.9, x64)	3m 10s
Windows (3.8, x64)	5m 46s
Windows (3.9, x86)	8m 8s
	1h 18m 24s

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