formatting empty where clauses when there are return types

[mu001999]

Fixes the bug, see rust-lang/rust#125242 (comment)

[ytmimi]

Given that rustfmt's current behavior is to leave empty where clauses as is, this change would need to be version gated.

I'm also curious how this change impacts comments before and after empty where clauses. For example:

fn foo() -> () /* comment */ where {}
fn bar() -> () where /* comment */ {}