feat: add different text when no PRs need review

Currently when no pull requests were needing review, the message would
show the same header, mentioning that there are pull requests to be
reviewed, followed by an empy list.

This commit fixes that and adds its own message stating that there are
no pull requests waiting for review.

lib/server.js

@@ -62,6 +62,12 @@ module.exports = function server () {
   }
 
   function buildMessage (data) {
+    if (data.length < 1) {
+      return Promise.resolve(
+        ':oncoming_police_car: No pull requests are waiting for review! :tada:'
+      )
+    }
+
     const headers = [
       ':police_car: The following pull requests are waiting for a review:',
       '\n'