Add FilterOptions::RETURN_ON_NULL

Author: chadicus

src/FilterOptions.php

@@ -24,6 +24,11 @@ final class FilterOptions
      */
     const IS_REQUIRED = 'required';
 
+    /**
+     * @var string
+     */
+    const RETURN_ON_NULL = 'returnOnNull';
+
     /**
      * @var string
      */