Fix Cerberus error

Same error (and same fix) as was fixed in Pillar 7dc0cadc.

flamenco/managers/patch.py

@@ -100,7 +100,7 @@ def patch_edit_from_web(self, manager_id: bson.ObjectId, patch: dict):
         self.log.info('User %s edits Manager %s: %s', current_user_id(), manager_id, update)
 
         validator = current_app.validator_for_resource('flamenco_managers')
-        if not validator.validate_update(update, manager_id):
+        if not validator.validate_update(update, manager_id, persisted_document={}):
             resp = jsonify({
                 '_errors': validator.errors,
                 '_message': ', '.join(f'{field}: {error}'