Mark return type of `as` operator as nullable
Mark return type of `as` operator as nullable

Fixes bug 612382.

playya/vala,kamanashisroy/aroop,frida/vala,frida/vala,spidermonkdust/vala,themiwi/vala,GNOME/vala,GNOME/vala,rvu95/vala,rvu95/vala,themiwi/vala,playya/vala,kamanashisroy/aroop,Distrotech/vala,flobrosch/vala-fixes,Distrotech/vala,rvu95/vala,severinh/vala,frida/vala,severinh/vala,playya/vala,spidermonkdust/vala,GNOME/vala,severinh/vala,themiwi/vala,frida/vala,tjol/vala-tjol,flobrosch/vala-fixes,tjol/vala-tjol,jessevdk/vala,tjol/vala-tjol,jessevdk/vala,GNOME/vala,frida/vala,flobrosch/vala-fixes,xdave/pvala,spidermonkdust/vala,jessevdk/vala,Distrotech/vala,kamanashisroy/aroop