The global post is not used if the post_excerpt is already set.
Change it to empty string to force post_excerpt generation via
the global post.

It now ensures wp_reset_postdata is correctly called

Inspired by the WordPress core test
https://github.com/WordPress/wordpress-develop/blob/0ad7b1a1695e3038ec14f3911291b69717c9a2ac/tests/phpunit/tests/query/setupPostdata.php#L150

Setting it up in resolve_post_object is too late because it fires before
the resolvers as resolvers can be called with different posts.

This also ensures that the global post is reset correctly after the
request.