Rollup merge of #84390 - m-ou-se:make-debug-non-exhaustive-without-fields-a-little...

Format `Struct { .. }` on one line even with `{:#?}`.

The result of `debug_struct("A").finish_non_exhaustive()` before this change:
```
A {
    ..
}
```
And after this change:
```
A { .. }
```

If there's any fields, the result stays unchanged:
```
A {
    field: value,
    ..
}