fix: preserve attributes for fn params with empty idents

[CPunisher]

fixes https://github.com/rust-lang/rustfmt/issues/6561 fixes https://github.com/rust-lang/rustfmt/issues/6607

just follow the suggestion and add some tests :)

[ytmimi]

Thanks for working on this. Can you also add a test case for https://github.com/rust-lang/rustfmt/issues/6607. I believe this also fixes that issue

[CPunisher]

@ytmimi I think this pr can be merged now

[av-gal]

This looks good to me.