`DataColumn<Nothing>.isNumber()` gives `true`

[Jolanrensen]

This breaks functions like describe() which calculate the mean() and std() for isNumber columns.

It's similar to #724 and isComparable().

[koperagen]

Small notice: seems strange that type is Nothing. Type of null should be Nothing?

[Jolanrensen]

Actually, technically speaking DataColumn<Nothing>.isNumber() should result in true. It's the fault of mean() and std() for not being able to give a result for an empty column. This result should likely be NaN instead of throwing an exception.