xcb: Replies are nullable and errors are optional
xcb: Replies are nullable and errors are optional

Also a few whitespace consistency changes thrown in.

frida/vala,GNOME/vala,spidermonkdust/vala,flobrosch/vala-fixes,spidermonkdust/vala,flobrosch/vala-fixes,frida/vala,frida/vala,frida/vala,frida/vala,GNOME/vala,rvu95/vala,GNOME/vala,flobrosch/vala-fixes,spidermonkdust/vala,GNOME/vala,rvu95/vala,rvu95/vala