Do not combine group_vars with an empty file

This addresses a specific case with multiple vars files
in a group_vars/${groupname}/ directory where one of those files
is empty, which returns None instead of an empty dict.

lib/ansible/vars/__init__.py

@@ -308,7 +308,8 @@ def _load_inventory_file(self, path, loader):
             paths = [os.path.join(path, name) for name in names if not name.startswith('.')]
             for p in paths:
                 _found, results = self._load_inventory_file(path=p, loader=loader)
-                data = self._combine_vars(data, results)
+                if results is not None:
+                    data = self._combine_vars(data, results)
 
         else:
             file_name, ext = os.path.splitext(path)