Update ExprBuilder.php

adapt to @return types of all others if* functions.

Author: gassan

src/Symfony/Component/Config/Definition/Builder/ExprBuilder.php

@@ -91,7 +91,7 @@ public function ifNull()
     /**
      * Tests if the value is empty.
      *
-     * @return ExprBuilder
+     * @return $this
      */
     public function ifEmpty()
     {