a11y: Use fallback logic when GApplication has no DBus object path

Use the fallback logic to generate the base path for the GtkAtSpiRoot
if the GApplication has no DBus object path to guarantee that
the base path will not stay NULL.

gtk/a11y/gtkatspiroot.c

@@ -690,7 +690,8 @@ gtk_at_spi_root_constructed (GObject *gobject)
       /* No need to validate the path */
       self->base_path = g_strconcat (app_path, "/a11y", NULL);
     }
-  else
+
+  if (self->base_path == NULL)
     {
       const char *program_name = g_get_prgname ();