glib: make the parameter of VariantDict constructor nullable
glib: make the parameter of VariantDict constructor nullable

The underlying glib implementation accepts null to initialize an empty
dictionary. The vapi now reflects this, defaulting the single parameter
to null.

Fixes bug 734612

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