Added missing impute_nan for WenAlloys and removed unnecessary warning

[gbrunin]

Summary

This fixes 2 small things:

• A warning about NaN imputation was written in case ElementProperty was initialiazed with impute_nan=False, even though the data source could be itself initialized with impute_nan=True. The warning is removed, since the imputation is only happening at the level of the data source.
• I missed an imputation in WenAlloys featurizer.

I'm make some tests now to decide whether the default of impute_nan should be switched to True.

[ml-evs]

👍