Validate data attribute

[ned2]

Currently, when the data attribute is set to None, Dash will happily accept as a valid value but this will result in the dreaded "Error loading dependencies" in the app.

This came up over in plotly/dash#616, where it resulted in a bug that was hard to pin down.

It would be nice if the validation could extend to values passed in by the callback, rather than just on the initial contents of the layout (although I don't know if we've solved this problem for validation of components elsewhere) as this was how the linked example manifested.