ci/github-script/bot: fix needs reviewer label

The recent change to use the result of requesting reviewers for setting
the `needs: reviewer` label caused a regression: It would not set the
label for PRs where no reviewers were requested, because *too many were
eligible*. Still - these PRs don't have reviewers, so they need
attention otherwise - via the label.

ci/github-script/reviewers.js

@@ -64,14 +64,6 @@ async function handleReviewers({
   ).filter(Boolean)
   log('reviewers - reviewers', reviewers.join(', '))
 
-  if (reviewers.length > 15) {
-    log(
-      `Too many reviewers (${reviewers.join(', ')}), skipping review requests.`,
-    )
-    // false indicates, that we do have reviewers and don't need the "needs: reviewers" label.
-    return false
-  }
-
   const requested_reviewers = new Set(
     pull_request.requested_reviewers.map(({ login }) => login),
   )
@@ -88,6 +80,14 @@ async function handleReviewers({
     Array.from(existing_reviewers).join(', '),
   )
 
+  if (reviewers.length > 15) {
+    log(
+      `Too many reviewers (${reviewers.join(', ')}), skipping review requests.`,
+    )
+    // Return a boolean on whether the "needs: reviewers" label should be set.
+    return existing_reviewers.size === 0 && requested_reviewers.size === 0
+  }
+
   const non_requested_reviewers = new Set(reviewers)
     .difference(requested_reviewers)
     // We don't want to rerequest reviews from people who already reviewed.
@@ -117,7 +117,7 @@ async function handleReviewers({
 
   // Return a boolean on whether the "needs: reviewers" label should be set.
   return (
-    new_reviewers.size === 0 &&
+    non_requested_reviewers.size === 0 &&
     existing_reviewers.size === 0 &&
     requested_reviewers.size === 0
   )