Don’t print a space after an empty struct name with `Formatter::debug_struct`

I wish I can print a structure like an anonymous structure by specifying an empty structure name using `Formatter::debug_struct`, for example:

```rust
struct Item;

impl Debug for Item {
    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
        f.debug_struct("").field("x", &2).finish()
    }
}
```

I expect `format!("{:?}", Item)` to be `"{ x: 2 }"`, instead, I get `" { x: 2 }"`.