`ContinuousEncoder` will drop columns with a scitype of `Union{Missing,Continuous}`

[ablaom]

This behaviour is probably unexpected. At least it caught me by surprise. I dislike issuing warnings, but I can't immediately think of a different mitigation.

[ablaom]

On further reflection, I think we can just leave Union{Continuous, Missing} alone, instead of dropping, and call this a bug fix.