How do we standardize arrays with NaN?
This is no longer the case; as of sklearn 0.20.0, missing values are ignored in such preprocessors' fit and silently passed along in their transform:
https://scikit-learn.org/stable/whats_new/v0.20.html#id37 (fourth bullet)
https://github.com/scikit-learn/scikit-learn/issues/10404