fix(migration): allow enrollmentflow's transit agency to be null, all…

…ow blanks for admin

benefits/core/migrations/0028_refactor_transitagency_enrollmentflow_relationship.py

@@ -23,7 +23,9 @@ class Migration(migrations.Migration):
         migrations.AddField(
             model_name="enrollmentflow",
             name="transit_agency",
-            field=models.ForeignKey(on_delete=django.db.models.deletion.PROTECT, to="core.transitagency"),
+            field=models.ForeignKey(
+                blank=True, null=True, on_delete=django.db.models.deletion.PROTECT, to="core.transitagency"
+            ),
             preserve_default=False,
         ),
         migrations.RunPython(migrate_data),