Fix: Check for the existence of the key. (Redmine issue: #6227)

lib/mentat/plugin/app/mailer.py

@@ -207,7 +207,7 @@ class MailerPlugin(pyzenkit.baseapp.ZenAppPlugin):
             )
             email_headers['to'] = self.get_application().c(self.CONFIG_MAIL_ADMIN)
 
-            if email_headers['cc']:
+            if 'cc' in email_headers and email_headers['cc']:
                 self.get_application().logger.info(
                     "Modifying report {}: header cc changed from {} to [] (rdr: {}, mtm: {})".format(
                         email_headers.get('report_id', 'None'),