`format_code_in_doc_comments` adds empty line between trailing comment and hidden code

```rust
/// ```
/// println!("1"); // comment
/// # println!("2")
/// ```
```

becomes

```rust
/// ```
/// println!("1"); // comment
///
/// # println!("2")
/// ```
```

rustfmt 1.8.0-nightly (9748d87dc7 2025-07-21)

Parent: #3348