Is your feature request related to a problem? Please describe.
Right now there's no efficient way to do a dot product that skips over nan elements.

Describe the solution you'd like
I want to be able to treat the summation in dot as a nansum, controlled by a skipna option. Either this can be implemented directly, or an additional ufunc can be added: xarray.unfuncs.nan_to_num that can be called on the inputs to dot. Unfortunately using numpy's nan_to_num will initiate eager execution.

Describe alternatives you've considered
It's possible to implement this by hand, but it ends up being extremely inefficient in one of my use-cases:

• (x*y).sum('dot_prod_dim', skipna=True) takes 30 seconds
• x.dot(y) takes 1 second