arg: Work around segfault in mozjs
gjs_value_to_g_argument() currently triggers a segfault in JS_IsUint8Array() when trying to convert the "feature" argument of tp_proxy_prepare_async() (a 0-terminated array of GQuark).
As a quick fix, swap around the conditions so that we don't hit the code path in case where we aren't actually trying to handle a ByteArray argument.