[bug 1068572] KB dashboard overview readout

r?
I want to clarify this a little.

First, we need `null=True` since we need to add the column via a migration and not all revisions will have it and that's the only way to add a new column.

Second, the `blank=True` tells form validation code that the field is not required and can be left blank. If you don't have `blank=True`, then forms based on that model will require a value for that field including the Django admin.