[backport 2.3.x] BUG (string dtype): let fillna with invalid value up… #20395

Job	Run time
Linux-32-bit	20m 15s
Linux-Musl	17m 54s
python-dev (macOS-13)	32m 14s
python-dev (macOS-14)	17m 18s
python-dev (ubuntu-22.04)	24m 16s
python-dev (windows-latest)	27m 3s
macos-13 actions-310.yaml	50m 32s
macos-13 actions-311.yaml	53m 43s
macos-13 actions-312.yaml	56m 29s
macos-13 actions-39.yaml	47m 18s
macos-14 actions-310.yaml	27m 44s
macos-14 actions-311.yaml	27m 6s
macos-14 actions-312.yaml	28m 40s
macos-14 actions-39.yaml	33m 19s
windows-latest actions-311.yaml	41m 30s
windows-latest actions-312.yaml	46m 30s
windows-latest actions-310.yaml	41m 45s
windows-latest actions-39.yaml	37m 20s
Copy-on-Write 3.10 (warnings)	29m 42s
ubuntu-latest actions-310.yaml	52m 37s
Copy-on-Write 3.10	31m 9s
ubuntu-latest actions-311.yaml	48m 34s
Future infer strings (without pyarrow)	47m 36s
Locale: zh_CN	28m 17s
Copy-on-Write 3.11	30m 12s
Copy-on-Write 3.11 (warnings)	29m 17s
Locale: it_IT	27m 47s
Downstream Compat	5m 44s
Numpy Dev	24m 35s
Pyarrow Nightly	25m 2s
ubuntu-latest actions-312.yaml	37m 28s
Future infer strings	41m 15s
Copy-on-Write 3.12	31m 25s
ubuntu-latest actions-39.yaml	51m 49s
Copy-on-Write 3.9 (warnings)	29m 48s
Copy-on-Write 3.9	28m 42s
Minimum Versions	28m 28s
Pypy	8m 27s
	21h 8m 50s

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