sd-bus: fix response for GetAll on non-existent objects

Before this commit, if you run `loginctl user-status` from
debug-shell.service (and you have no login sessions for root), you always
see this output:

0
	Linger: no

because Properties.GetAll is returning success but without any properties,
when the only find() callback had returned 0 to mean "no object found".

After:

Could not get properties: Unknown object:
'/org/freedesktop/login1/user/self'


BTW I have a fix for more user-friendly messages from logind in this case.
It is pending in my local branch for #6829 "fix `loginctl enable-linger`".

src/libsystemd/sd-bus/bus-objects.c

@@ -829,6 +829,9 @@ static int property_get_all_callbacks_run(
                         return 0;
         }
 
+        if (!*found_object)
+                return 0;
+
         if (!found_interface) {
                 r = sd_bus_reply_method_errorf(
                                 m,