Editor: Safeguard has_blocks() against fatal errors.

This changeset ensures `has_blocks()` doesn't return a fatal error when `$post` is not a valid post. If the post can't be retrieved, the function now returns `false`.

Props Howdy_McGee, costdev, colonelphantom, audrasjb, dlh, peterwilsoncc.
Fixes #55705.


git-svn-id: https://develop.svn.wordpress.org/trunk@53858 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/blocks.php

@@ -425,9 +425,12 @@ function unregister_block_type( $name ) {
 function has_blocks( $post = null ) {
 	if ( ! is_string( $post ) ) {
 		$wp_post = get_post( $post );
-		if ( $wp_post instanceof WP_Post ) {
-			$post = $wp_post->post_content;
+
+		if ( ! $wp_post instanceof WP_Post ) {
+			return false;
 		}
+
+		$post = $wp_post->post_content;
 	}
 
 	return false !== strpos( (string) $post, '<!-- wp:' );