Fixed #23405 - Fixed asking for default value for `CharField` and `TextField` in migrations

I think modifying autodetector will be enough.
When migration will be applying, `django.db.backends.schema.BaseDatabaseSchemaEditor#effective_default` set correct value (empty string) for `TextField` and `CharField`, if they are blank, but NOT NULL.
We don't usually reference tickets in code comments, as it quickly leads to accumulated clutter. `git blame` can be used to track down the reasoning for a change, when necessary.