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.
minor edit:
```
* ``makemigrations`` no longer prompts for a default value when adding
  ``TextField()`` or ``CharField()`` without a default (:ticket:`23405`).
```