typing: refuse a null value in superstrings

Signed-off-by: Jean Privat <jean@pryen.org>
tests: test new policy in test_superstring

Signed-off-by: Jean Privat <jean@pryen.org>
update code to comply with new superstring policy

Signed-off-by: Jean Privat <jean@pryen.org>
Merge: Refuse null in superstrings

Fixes #540

Pull-Request: #542
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>