hub: simplify recipients condition in build_notification()

If get_notification_recipients() returns a non-empty list of recipients,
then the build_notification() method creates a new buildNotification task.

Simplify the conditional that checks the get_notification_recipients()
return value so that it is easier to read. Python treats an empty list
as False and a non-empty list as True, so we don't need to explicitly
test the list length here.

The purpose of this change is to make this code easier to read and
understand.

hub/kojihub.py

@@ -8457,7 +8457,7 @@ def build_notification(task_id, build_id):
     web_url = context.opts.get('KojiWebURL', 'http://localhost/koji')
 
     recipients = get_notification_recipients(build, dest_tag, build['state'])
-    if len(recipients) > 0:
+    if recipients:
         make_task('buildNotification', [recipients, build, target, web_url])