Bug 621119 – GDBusProxy and objects with no properties

Fix proxy construction for objects with no properties in the case
where G_DBUS_PROXY_FLAGS_DO_NOT_LOAD_PROPERTIES isn't set.

The unfortunate side-effect here is that GDBusProxy can no longer be
used to test for "object existence", e.g. creating a GDBusProxy for
any path and interface will not fail. But that's not really a big
deal, if apps rely on that they are doing something very wrong.

https://bugzilla.gnome.org/show_bug.cgi?id=621119



Signed-off-by: David Zeuthen <davidz@redhat.com>