Bug: Crash on `{} => true`

The following causes a crash

```ts
import { Parser } from 'n3';
import { write } from '@jeswr/pretty-turtle';

const parser = new Parser({ format: 'text/n3' });
write(parser.parse('{} => true .'), { format: 'text/n3' }).then(console.log)
```

Start by running the TypeScript reproduction in the issue with n3's Parser and @jeswr/pretty-turtle's write function, then trace where the {} => true input causes the crash. Done means this valid-looking input no longer crashes and its resulting behavior is covered by a regression test or otherwise verified.

Written by the indexing model from the issue text.