Return the external issue tracker even if it's null

This solves the problem with caching the nil value with instance variable.
Without this the every time we ask for external_issue_tracker we built AR and potentially do SQL query

app/models/project.rb

@@ -508,6 +508,7 @@ class Project < ActiveRecord::Base
   end
   def external_issue_tracker
+    return @external_issue_tracker if defined?(@external_issue_tracker)
     @external_issue_tracker ||=
       services.issue_trackers.active.without_defaults.first
   end