It would always be initialized but initialize it to NULL to silence the
compiler, and also check that it is not NULL anymore when we expect it
to contain a valid value.

../gio/tests/desktop-app-info.c: In function ‘test_fallback’:
../gio/tests/desktop-app-info.c:191:18: warning: ‘app’ may be used uninitialized in this function [-Wmaybe-uninitialized]
   g_assert_true (g_app_info_equal (info1, app));
                  ^~~~~~~~~~~~~~~~