pandas.unique needs to be more careful on index types #1759
Description
e.g.
In [2]: mindex = MultiIndex.from_arrays([np.arange(5).repeat(5), np.tile(np.arange(5), 5)])
In [3]: pd.unique(mindex)
Out[3]: array([], dtype=object)
{{ message }}
e.g.
In [2]: mindex = MultiIndex.from_arrays([np.arange(5).repeat(5), np.tile(np.arange(5), 5)])
In [3]: pd.unique(mindex)
Out[3]: array([], dtype=object)