TYP: fix single-arg ma.where return type - #31919

Single-arg np.ma.where calls return a tuple of ndarray, not MaskedArray:

In [1]: np.ma.where(np.array([True, False]))
Out[1]: (array([0]),)

In [2]: np.ma.where(np.array([True, False]).view(np.ma.MaskedArray))
Out[2]: (array([0]),)

Fully written by me (a human); bug found by Claude Fable 5 (here's the full (second) audit for those interested).

(even more stub fixes on the way)