'liechtenstein' Hubbard formulation

When trying to set the Hubbard formulation to 'liechtenstein' in an instance of the `Hubbard` class, I'm getting 

```bash
File ".../aiida_quantumespresso/utils/hubbard.py", line 69, in get_hubbard_card
    line = f'{pre}\t{atom_i}-{man_i} \t{value}'  # noqa: F821
              ^^^
UnboundLocalError: cannot access local variable 'pre' where it is not associated with a value
```

Version 4.16.0