Core's config/auth/list WS command always returns a list, never None.
Replace the silent `if not raw: return []` (which also swallowed empty
lists) with an assert, remove the dead AuthListUsersNoneResponseError
exception class, and document the HomeAssistantWSError contract in the
docstring.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>