[Xwt.Gtk] Put in a workaround for broken GdkEvent handling in gtk-sharp

Null GdkEvent objects are supposed to be marshalled to IntPtr.Zero
but the gtk-sharp binding generator generates the code as:
'event.Handle' instead of 'event == null ? IntPtr.Zero : event.Handle'

This is a trivial workaround for the problem which has no ill effects.