[bug 1068572] KB dashboard overview readout

r?
A different thing: `null=True`, I recently learned, doesn't mean "this field can be blank". It means "this field, if it is blank, will be stored as a null value." I hit this a bunch with the DRF questions/answers API. I think what you want here is `blank=True` instead. This is true almost everywhere we use `null=True`, I think.