Post Thumbnails: Change the return value of get_post_thumbnail_id() for a non-existing post to false instead of an empty string.

This further makes the function more consistent with `get_the_ID()` or `wp_get_post_parent_id()`, both returning `false` for a non-existing post.

Additionally, document that `get_post_thumbnail_id()` returns `0` if the thumbnail image is not set.

Follow-up to [47160].

Props theMikeD, dingo_d, netpassprodsr, SergeyBiryukov.
Fixes #49832. See #40096.

git-svn-id: https://develop.svn.wordpress.org/trunk@48310 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/post.php

@@ -7014,7 +7014,8 @@ function _publish_post_hook( $post_id ) {
  * @since 3.1.0
  *
  * @param int|WP_Post $post Post ID or post object. Defaults to global $post.
- * @return int|false Post parent ID (which can be 0 if there is no parent), or false if the post does not exist.
+ * @return int|false Post parent ID (which can be 0 if there is no parent),
+ *                   or false if the post does not exist.
  */
 function wp_get_post_parent_id( $post ) {
 	$post = get_post( $post );