Argh, this is legacy stuff that I dealt with "on the safe side". This check was performed beforehand and I add the None check after the decision was taken to have `None` instead of `1.0` be the default.

The line https://github.com/scikit-learn/scikit-learn/pull/3034/files#diff-489e81d5edaa5bb1d352988699773551R259 converts all `None` to `1.0`. I will take care to refactor this properly.