cairo: Mark filename parameter for PdfSurface nullable
cairo: Mark filename parameter for PdfSurface nullable

Fixes bug 644158.

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