GDBusConnection: be more careful with async GetAll

It's possible to get a org.freedesktop.Properties.GetAll call even if we
have no readable properties in the introspection, in which case we
should return the empty list in the usual way.

We should certainly _not_ be dispatching to the method call handler of
an interface which has no properties (since it will not be expecting
this).

Add a check to make sure that there is at least one readable property
before assuming that a NULL get_property handler implies that we want to
handle properties asynchronously.

Add a testcase that was failing before the change and works after it.

https://bugzilla.gnome.org/show_bug.cgi?id=703437