Generic unit variants are handled incorrectly inside format args

The following is a valid format expression, but thiserror fails to compile it.

```rust
use thiserror::Error;

#[derive(Debug, Error)]
#[error("{}", None::.is_some())]
pub struct Error;

fn main() {
println!("{}", None::.is_some());
}
```

```console
error: expected one of `!`, `,`, `.`, `::`, `?`, `{`, or an operator, found `is_some`
--> src/main.rs:4:27
|
4 | #[error("{}", None::.is_some())]
| ^^^^^^^ expected one of 7 possible tokens
```