Make response_type an optional string if initialized to None

This reduced mypy type errors from 354 to 163.

Author: agners

src/controller/python/templates/python-cluster-Objects-py.zapt

@@ -121,7 +121,7 @@ class {{asUpperCamelCase name}}(Cluster):
             {{~#if responseName}}
             response_type: typing.ClassVar[str] = '{{asUpperCamelCase responseName}}'
             {{else}}
-            response_type: typing.ClassVar[str] = None
+            response_type: typing.ClassVar[typing.Optional[str]] = None
             {{/if}}
 
             @ChipUtility.classproperty