BUG: Fix pivot_table margins to include NaN groups when dropna=False #23611

Job	Run time
Linux-Musl	14m 56s
Linux-32-bit	18m 10s
macos-14 actions-310.yaml	25m 50s
macos-14 actions-312.yaml	23m 2s
windows-latest actions-311.yaml	1h 25m 50s
Pyodide build	25m 18s
macos-14 actions-313.yaml	25m 10s
windows-latest actions-310.yaml	1h 23m 2s
windows-latest actions-313.yaml	1h 30m 43s
macos-13 actions-312.yaml	53m 23s
python-dev	0s
macos-13 actions-310.yaml	59m 19s
macos-13 actions-311.yaml	50m 39s
macos-14 actions-311.yaml	24m 10s
macos-13 actions-313.yaml	57m 24s
windows-latest actions-312.yaml	1h 20m 20s
Future infer strings	1h 25m 7s
Locale: it_IT	1h 4m 39s
ubuntu-24.04-arm actions-312.yaml	59m 41s
ubuntu-24.04-arm actions-311.yaml	58m 52s
Numpy Dev	17m 39s
ubuntu-24.04 actions-311.yaml	1h 22m 9s
Minimum Versions	50m 3s
ubuntu-24.04 actions-312.yaml	1h 25m 35s
Freethreading	21m 28s
Pypy	1m 39s
ubuntu-24.04-arm actions-310.yaml	58m 46s
Downstream Compat	6m 0s
ubuntu-24.04 actions-313.yaml	1h 30m 23s
Locale: zh_CN	1h 4m 35s
ubuntu-24.04 actions-310.yaml	1h 27m 46s
Pyarrow Nightly	19m 58s
ubuntu-24.04-arm actions-313.yaml	1h 4m 11s
Future infer strings (without pyarrow)	1h 19m 8s
	1d 4h 34m 55s

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