Whether the viewer has liked the post, or None if not logged in.

def viewer_has_liked(self) -> Optional[bool]:
        """Whether the viewer has liked the post, or None if not logged in."""
        if not self._context.is_logged_in:
            return None
        if 'likes' in self._node and 'viewer_has_liked' in self._node['likes']:
            return self._node['likes']['viewer_has_liked']
        return self._field('viewer_has_liked')