Avoid running cypher statement if not needed

- it just feels safer to not run a cypher statement if there is no user
  in the idsOfUsers array. Why risk notifications pointing to empty
nodes? It doesn't seem like this happens, but I'm not sure how to write
a test to verify, and why run cypher statements, if there is no need to?

backend/src/middleware/notifications/notificationsMiddleware.js

@@ -53,6 +53,7 @@ const postAuthorOfComment = async (commentId, { context }) => {
 }
 
 const notifyUsersOfMention = async (label, id, idsOfUsers, reason, context) => {
+  if (!(idsOfUsers && idsOfUsers.length)) return []
   await validateNotifyUsers(label, reason)
   let mentionedCypher
   switch (reason) {