Add a function to return an empty string, for filters. props wpsmith, trepmal. fixes #20357.

git-svn-id: https://develop.svn.wordpress.org/trunk@25037 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/functions.php

@@ -3667,6 +3667,19 @@ function __return_null() {
 	return null;
 }
 
+/**
+ * Returns an empty string.
+ *
+ * Useful for returning an empty string to filters easily.
+ *
+ * @since 3.7.0
+ * @see __return_null()
+ * @return string Empty string
+ */
+function __return_empty_string() {
+	return '';
+}
+
 /**
  * Send a HTTP header to disable content type sniffing in browsers which support it.
  *