Themes: Fix fatal error in load-styles.php.

Following [56635], a fatal error occurred in `load-styles.php` leading to admin styles not working, because of a `has_filter()` call being added to `get_stylesheet_director()` and `get_template_directory()`.

This changeset adds `has_filter()` to `wp-admin/includes/noop.php` to prevent such errors. The lack of loading the function does not cause any unintended side effects itself.

Props iandunn, adamsilverstein.
Fixes #59417.
See #18298.


git-svn-id: https://develop.svn.wordpress.org/trunk@56641 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/noop.php

@@ -22,6 +22,13 @@ function _x() {}
  */
 function add_filter() {}
 
+/**
+ * @ignore
+ */
+function has_filter() {
+	return false;
+}
+
 /**
  * @ignore
  */