Incorrect formatting of -0

In various circumstances, -0 has an extra plus sign appended where it should not:

```rs
use num::complex::Complex;

fn main() {
    println!("{}", Complex::new(-0.0, -0.0));
    println!("{:+}", Complex::new(-0.0, -0.0));
}
```
```
-0+-0i
+-0+-0i
```

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2024&gist=c5368ca99596e43a2f3a59e7762e396c