Add possibility to show a message whenever any widget has no data

Currently, whenever a widget has no data to display, we just get a blank chart with empty axes:

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Which is ok, but not super-intuitive, and leads to people saying “omg the dashboard is broken”, because they didn’t realize that the date range they selected is in the future, or stuff like that.

So I was thinking it would be nice to have the possibility of overlaying those empty visualizations with a message (possibly customizable). Something like:

image