Fix GIR of VteTerminalSpawnAsyncCallback
Otherwise the Vte.Terminal.spawn_async function is unusable from Vala, as it adds an g_return_if_null (error) at the top of the function, but if it is successful, error is defined to be null => Bug
Otherwise the Vte.Terminal.spawn_async function is unusable from Vala, as it adds an g_return_if_null (error) at the top of the function, but if it is successful, error is defined to be null => Bug