Add `__return_null()` to complement the existing true, false, zero, and empty array methods. props johnbillion, fixes #18845.

git-svn-id: https://develop.svn.wordpress.org/trunk@19607 602fd350-edb4-49c9-b593-d223f7449a82

wp-includes/functions.php

@@ -3501,6 +3501,18 @@ function __return_empty_array() {
 	return array();
 }
 
+/**
+ * Returns null.
+ *
+ * Useful for returning null to filters easily.
+ *
+ * @since 3.3.0
+ * @return null
+ */
+function __return_null() {
+	return null;
+}
+
 /**
  * Send a HTTP header to disable content type sniffing in browsers which support it.
  *