Empty labels should not be allowed

### Description

Attaching an empty label to a figure (or other element) as in
```typ
#figure(rect(), caption: [...]) #label("")
```
allows you to reference it with just `@`, which should probably not be allowed.

### Reproduction URL

_No response_

### Operating system

Web app, Windows

### Typst version

- [x] I am using the latest version of Typst