Tunnel end fallback syntax

I want to silently end a tunnel if there are no choices available (i.e. fallback to ->-> with no output).

```
// Doesn't work
* ->->

// Doesn't work
* [] ->->

// Does work, but is really hacky
* [] <>
->->
```

Apparently, fallbacks require either text or a divert arrow, and glue counts as text. Is there any better syntax for this? Note that I don't want to use a gather, because I want the tunnel to continue if there are choices available.