Remove pretty printer space inside block with only outer attrs

[dtolnay]

Follow-up to #92238 fixing one of the FIXMEs.

macro_rules! repro {
    ($expr:expr) => {
        stringify!($expr)
    };
}

fn main() {
    println!("{}", repro!(#[attr] {}));
}

Before: #[attr] { }
After: #[attr] {}

[rust-highfive]

r? @oli-obk

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[oli-obk]

@bors r+ rollup

[bors]

📌 Commit cbccc4a has been approved by oli-obk