allow defaults with no include vars (#6613)

esphome/yaml_util.py

@@ -321,7 +321,8 @@ class ESPHomeLoaderMixin:
             file, vars = node.value, None
 
         result = _load_yaml_internal(self._rel_path(file))
-        if vars:
+        if not vars:
+            vars = {}
         result = substitute_vars(result, vars)
         return result