DOC: Whatsnew for DataFrameGroupBy.__len__ with NA values bugfix (#58742)

* DOC: Whatsnew for DataFrameGroupBy.__len__ with NA values bugfix

* Update doc/source/whatsnew/v3.0.0.rst

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Co-authored-by: Matthew Roeschke <[email protected]>

Author: rhshadrach

doc/source/whatsnew/v3.0.0.rst

@@ -469,6 +469,7 @@ Groupby/resample/rolling
 - Bug in :meth:`DataFrameGroupBy.apply` that was returning a completely empty DataFrame when all return values of ``func`` were ``None`` instead of returning an empty DataFrame with the original columns and dtypes. (:issue:`57775`)
 - Bug in :meth:`DataFrameGroupBy.apply` with ``as_index=False`` that was returning :class:`MultiIndex` instead of returning :class:`Index`. (:issue:`58291`)
 - Bug in :meth:`DataFrameGroupby.transform` and :meth:`SeriesGroupby.transform` with a reducer and ``observed=False`` that coerces dtype to float when there are unobserved categories. (:issue:`55326`)
+- Bug in :meth:`.DataFrameGroupBy.__len__` and :meth:`.SeriesGroupBy.__len__` would raise when the grouping contained NA values and ``dropna=False`` (:issue:`58644`)
 
 
 Reshaping