Allow `DataFrame.aggregate` to accept `None` for no grouping #527

Job	Run time
build / lint-rust	1m 58s
build / generate-license	1m 58s
build / lint-python	9s
build / lint-toml	4s
build / check-crates-patch	-9s
build / Linux arm64 (3.13t)	11m 43s
build / Linux arm64 (3.14t)	11m 55s
build / Linux x86_64 (3.14t)	13m 59s
build / Linux arm64 (abi3)	11m 38s
build / Linux x86_64 (3.13t)	14m 4s
build / Linux x86_64 (abi3)	20m 2s
build / Windows x86_64 (3.14t)	14m 25s
build / macOS arm64 (3.13t)	8m 55s
build / macOS arm64 (3.14t)	8m 54s
build / macOS arm64 (abi3)	9m 55s
build / Windows x86_64 (3.13t)	13m 51s
build / Windows x86_64 (abi3)	14m 18s
build / Source distribution	0s
build / macOS x86_64 (${{ matrix.python-tag }})	0s
build / Build docs	54s
build / Merge build artifacts	0s
test / test-matrix (3.13t, 3.13t, true)	1m 4s
test / test-matrix (3.14, abi3, false)	4m 33s
test / test-matrix (3.12, abi3, false)	3m 53s
test / test-matrix (3.10, abi3, false)	3m 59s
test / test-matrix (3.11, abi3, false)	3m 49s
test / test-matrix (3.14t, 3.14t, true)	2m 53s
test / test-matrix (3.13, abi3, false)	4m 4s
	3h 2m 48s

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