Update return type in docblock.

src/Symfony/Component/Console/Input/InputInterface.php

@@ -21,7 +21,7 @@ interface InputInterface
     /**
      * Returns the first argument from the raw parameters (not parsed).
      *
-     * @return string The value of the first argument or null otherwise
+     * @return string|null The value of the first argument or null otherwise
      */
     public function getFirstArgument();