ENH: support masked arrays in groupby cython algos (original) (raw)

Similarly as for normal reductions (eg #30982), we should investigate having masked array-specific support in the groupby algorithms.

Currently, when starting from a nullable extension array, they get converted to a numpy array (eg integers with missing values will typically get cast to float with nan) before passing to the cython algorithm.
Having support for passing a mask to the cython algos can improve the groupby support for nullable dtypes.