Fix problems when engelsystem_url is not set

app/jobs/pretalx/import_job.rb

@@ -115,7 +115,7 @@ module Pretalx
 
     def import_engelsystem_refs(conference)
       unless translation_angel_id = conference.fetch_translation_angel_id
-        logger.warn("Could not find translation angel id for #{conference.slug}")
+        Rails.logger.warn("Could not find translation angel id for #{conference.slug}")
         return
       end
       return unless data = conference.fetch_engelsystem("angeltypes/#{translation_angel_id}/shifts")

app/models/conference.rb

@@ -131,6 +131,7 @@ class Conference < ApplicationRecord
   end
 
   def fetch_engelsystem(endpoint)
+    return nil unless engelsystem_url
     endpoint_url = engelsystem_url + endpoint
     Rails.logger.debug("Querying engelsystem API at #{endpoint_url}")
     response = HTTParty.get(