Merge: Set accept null

Since keys of Maps can now be null, it enables the same rule with the Sets

Pull-Request: #1132
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>