Document with no abstract fails

The example at <https://typst.app/project/wGseT9HAhk-HHQ7oVBJ-pw> contains

```typst
#import "lapreprint.typ": template

#show: template.with(
  title: "A beautiful preprint template"
)
```

and fails with "Type none has no method map (line 278)"

This is related to #4 so suggestion is to do some kind of if-else logic to handle metadata that potentially does not exist.