Also always post_validate Handler class as well

Fixes #12154

lib/ansible/playbook/base.py

@@ -266,7 +266,7 @@ class Base:
                     continue
                 else:
                     raise AnsibleParserError("the field '%s' is required but was not set" % name)
-            elif not attribute.always_post_validate and self.__class__.__name__ not in ('Task', 'PlayContext'):
+            elif not attribute.always_post_validate and self.__class__.__name__ not in ('Task', 'Handler', 'PlayContext'):
                 # Intermediate objects like Play() won't have their fields validated by
                 # default, as their values are often inherited by other objects and validated
                 # later, so we don't want them to fail out early