Bug fix when "stores" is not present in YAML config

Resolves error when "stores" is returned as None, rather than an empty dictionary in the event no "stores" are present in the YAML configuration

ldms/python/ldmsd/parser_util.py

@@ -578,9 +578,9 @@ def build_stores(self, config):
         Return a dictionary keyed by the group name. Each dictionary
         entry is a list of stores in that group.
         """
-        if 'stores' not in config:
-            return None
         stores = {}
+        if 'stores' not in config:
+            return stores
         if type(config['stores']) is not dict:
             raise ValueError(f'{LDMS_YAML_ERR}\n'
                              f'store {DICT_ERR}\n'