Awkward spacing with `#[related]` errors with `#[diagnostic(code)]`s

[CAD97]

Example:

use {
    miette::{Diagnostic, SourceSpan},
    thiserror::Error,
};

#[derive(Debug, Error, Diagnostic)]
#[error("{} errors", .related.len())]
struct Errors {
    #[source_code]
    src: String,
    #[related]
    related: Vec<Error>,
}

#[derive(Debug, Error, Diagnostic)]
#[error("some error")]
#[diagnostic(code("example::error"))]
struct Error {
    #[label]
    span: SourceSpan,
}

fn main() -> miette::Result<()> {
    Err(Errors {
        src: "test test".into(),
        related: vec![
            Error {
                span: (0..4).into(),
            },
            Error {
                span: (5..9).into(),
            },
        ],
    })?;
    Ok(())
}

Error: 
  × 2 errors

Error: example::error

  × some error
   ╭────
 1 │ test test
   · ────
   ╰────
Error: example::error

  × some error
   ╭────
 1 │ test test
   ·      ────
   ╰────

The empty line between the error code and the rendered error causes the errors to visually group incorrectly; either diagnostics should never have any blank lines, or #[related] needs to put blank lines between the related diagnostics.

Without the diagnostic code, the blank line is not present and the visual grouping is fine:

Error:
  × 2 errors

Error:
  × some error
   ╭────
 1 │ test test
   · ────
   ╰────
Error:
  × some error
   ╭────
 1 │ test test
   ·      ────
   ╰────