Explicitly define SQL_QUERY_MUTATOR as passthrough function

PR #118 set SQL_QUERY_MUTATOR = None which caused 'NoneType' object is
not callable in the Celery sql_lab worker. The previous commit removed
that line, but explicitly defining the passthrough function ensures the
Superset config module loading path cannot re-introduce a None value
regardless of import order or config overlay behavior.

Author: szachovy

services/superset/superset_config.py

@@ -43,6 +43,12 @@ class CeleryConfig:  # pylint: disable=too-few-public-methods
 
 CELERY_CONFIG = CeleryConfig  # pylint: disable=invalid-name
 RESULTS_BACKEND = flask_caching.backends.rediscache.RedisCache(host="redis", port=6379, key_prefix="superset_results")
+
+
+def SQL_QUERY_MUTATOR(sql, **kwargs):  # pylint: disable=invalid-name,unused-argument
+    return sql
+
+
 SQL_MAX_ROW = 10000
 DEFAULT_SQLLAB_LIMIT = 1000
 SQLLAB_TIMEOUT = 300