relnotes is eating up empty lines

[Urgau]

I discovered in rust-lang/rust#135797 that relnotes was eating up empty lines from the relnote tracking issues.

The issue is that in Markdown empty lines are relevant as they allow for breaking the paragraph and starting a new one.

---

One can see the difference here:

• My dummy PR with an long long title for example
  No new lines between this line and the title above.

• My dummy PR with an long long title for example

  New line added between the title and this line.

---

I think the issue is due to this check,

relnotes/src/main.rs

Lines 323 to 325 in e5786bb

	if line.trim().is_empty() {
	continue;
	}

cf. https://rust-lang.zulipchat.com/#narrow/channel/241545-t-release/topic/1.2E85/near/499937064
cc @Mark-Simulacrum