Presenter::$layout can be either string or null

[mabar]

trafficstars

• bug fix? yes
• BC break? no

It was not really string|bool but string|false|null, which kinda surprised me when I was implementing own layout finder

This variant is the only one I found which simplifies problem and does not break default implementation. Not sure about custom ones.

Alternative variant is change of annotation to string|false|null, but it's ugly.