WP-r44338: Correctly show post type hierarchy in admin (#333)

In https://core.trac.wordpress.org/changeset/44185, a bug was introduced where hierarchical post types would not display in the correct default order (hierarchically).

This was caused by a `! isset()` check, which returned `false` after https://core.trac.wordpress.org/changeset/44185, causing the correct default value to not be applied. This switches that conditional to use an `empty()` check, ignoring the new empty string assignment that was added to prevent a PHP notice when `compact()` is called.

WP:Props davidbinda.
Fixes https://core.trac.wordpress.org/ticket/45711.

Conflicts:
src/wp-admin/includes/post.php
----
Merges https://core.trac.wordpress.org/changeset/44338 / WordPress/wordpress-develop@47584c7bc0 to ClassicPress.

See #263.

src/wp-admin/includes/post.php

@@ -1086,10 +1086,10 @@ function wp_edit_posts_query( $q = false ) {
 	$query = compact('post_type', 'post_status', 'perm', 'order', 'orderby', 'posts_per_page');
 
 	// Hierarchical types require special args.
-	if ( is_post_type_hierarchical( $post_type ) && !isset($orderby) ) {
-		$query['orderby'] = 'menu_order title';
-		$query['order'] = 'asc';
-		$query['posts_per_page'] = -1;
+	if ( is_post_type_hierarchical( $post_type ) && empty( $orderby ) ) {
+		$query['orderby']                = 'menu_order title';
+		$query['order']                  = 'asc';
+		$query['posts_per_page']         = -1;
 		$query['posts_per_archive_page'] = -1;
 		$query['fields'] = 'id=>parent';
 	}