Make DataFrame respect allow_None=True

[poplarShift]

Carrying this over from #404.

Should we validate the provided default? Currently, param is e.g. ok with param.Number(default=None, bounds=(1, 100)) but will throw an error at param.DataFrame(default=None, rows=(1, 10)).

Param usually sets allow_None=True if the default is set to None, under the assumption that the Parameterized class author clearly thus intends None to be valid for this Parameter. If param.DataFrame does not do that, I would be happy for such code to be added to param.DataFrame (and to these new classes) to do that; it seems like a reasonable thing to do for the convenience of authors.

It seems like DataFrame sets the allow_None flag accordingly but does not take it into account in _validate.

[jbednar]

Ah, that makes sense. Definitely a bug in param.Dataframe, then.