core: short-circuit nm_device_spec_match_list()

If there are no specs, then the device can't match, so don't call the
virtual method (which might do work like building comparison strings
even when the list is empty).

src/devices/nm-device.c

@@ -6347,6 +6347,9 @@ nm_device_spec_match_list (NMDevice *device, const GSList *specs)
 {
 	g_return_val_if_fail (NM_IS_DEVICE (device), FALSE);
 
+	if (!specs)
+		return FALSE;
+
 	return NM_DEVICE_GET_CLASS (device)->spec_match_list (device, specs);
 }