scanner: fix null check for optionnal arguments
scanner: fix null check for optionnal arguments

Actually check that the argument is a nullptr, and not the pointer
to the argument...

vberger/wayland-client-rs,vberger/wayland-client-rs,vberger/wayland-rs,Smithay/wayland-rs,Smithay/wayland-rs,vberger/wayland-client-rs,vberger/wayland-rs