Fix onceoff job to only run when slack is installed

app/jobs/onceoff/migrate_from_slack_official.rb

@@ -4,6 +4,7 @@ module Jobs
       # Check if slack plugin is installed by testing if the sitesetting exists
       slack_installed = defined? DiscourseSlack
 
+      if slack_installed
         already_setup_rules = DiscourseChat::Channel.with_provider('slack').exists?
 
         already_setup_sitesettings =
@@ -16,6 +17,7 @@ module Jobs
           migrate_settings()
           migrate_data()
         end
+      end
 
     end