Even if g_variant_get_va(), which we eventually call, checks on the
validity of the value argument, check it early to avoid surprises, such
as this intermittent FreeBSD test failure:
(/var/tmp/gitlab_runner/builds/3fe11159/0/GNOME/glib/_build/gio/tests/gdbus-connection:65788): GLib-CRITICAL **: 15:13:25.670: g_variant_get_va: assertion 'value != NULL' failed