Loader: Now ignores CELERY_CONFIG_MODULE if value is empty string

celery/loaders/base.py

@@ -240,8 +240,9 @@ class BaseLoader(object):
         except KeyError:
             pass
         else:
-            usercfg = self._import_config_module(custom_config)
-            return DictAttribute(usercfg)
+            if custom_config:
+                usercfg = self._import_config_module(custom_config)
+                return DictAttribute(usercfg)
         return {}
 
     @property