BUG: Fix pivot_table margins to include NaN groups when dropna=False #24412

Job	Run time
Linux-Musl	54s
Linux-32-bit	53s
python-dev	0s
Pyodide build	55s
macos-13 actions-313.yaml	49s
macos-14 actions-310.yaml	1m 7s
macos-14 actions-312.yaml	1m 8s
macos-13 actions-310.yaml	49s
windows-latest actions-311.yaml	56s
macos-14 actions-311.yaml	1m 9s
macos-13 actions-311.yaml	48s
macos-14 actions-313.yaml	1m 7s
windows-latest actions-312.yaml	56s
windows-latest actions-313.yaml	57s
macos-13 actions-312.yaml	48s
ubuntu-24.04 actions-311.yaml	56s
Downstream Compat	55s
Freethreading	56s
ubuntu-24.04-arm actions-310.yaml	55s
ubuntu-24.04 actions-313.yaml	56s
ubuntu-24.04-arm actions-312.yaml	54s
Locale: it_IT	56s
Locale: zh_CN	55s
Pyarrow Nightly	56s
ubuntu-24.04-arm actions-313.yaml	55s
ubuntu-24.04-arm actions-311.yaml	54s
windows-latest actions-310.yaml	56s
ubuntu-24.04 actions-312.yaml	55s
Future infer strings (without pyarrow)	55s
Minimum Versions	54s
Numpy Dev	55s
ubuntu-24.04 actions-310.yaml	55s
Future infer strings	57s
	29m 51s

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