Allow empty commit message

Although this is unlikely (or impossible?) to ever happen in practice,
I don't feel comfortable with crashing triagebot if it ever happens.

src/handlers/milestone_prs.rs

@@ -163,7 +163,7 @@ async fn milestone_cargo(
                 .message
                 .lines()
                 .next()
-                .expect("commit message");
+                .unwrap_or_default();
             merge_re.captures(first).map(|cap| {
                 cap.get(1)
                     .or_else(|| cap.get(2))