Unrwap on Option::None when no schema files are given

Calling planus-cli with `planus rust -o foo` (i.e. with not .fbs schema file(s))  will result in the panic:
```
The application panicked (crashed).
Message:  called `Option::unwrap()` on a `None` value
Location: /home/nicklas/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/planus-types-1.3.0/src/intermediate.rs:144
```