Support masked where in ufuncs

[mhvk]

This pull request is to address the case where a where argument in a ufunc is masked. For this case, any masked elements will be masked in the output as well. This is not very useful in itself, but avoids problems in conditional functions (like np.add(ma, 1, where=ma>10)). It also means np.fix will work also with numpy >= 1.25.

Fixes #14571

[github-actions]

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[mhvk]

@pllim - I think this is indeed ready to merge! Thanks, @byrdie, for the quick review.

[pllim]

Thanks!