Load 'Gitlab::Jira::Middleware' if it exists

Loads 'Gitlab::Jira::Middleware' only if it exists
so this initializer can be backported to CE.

config/initializers/jira.rb

-# Treats JIRA DVCS user agent requests in order to be successfully handled
+# frozen_string_literal: true
+
+# Changes JIRA DVCS user agent requests in order to be successfully handled
 # by our API.
-Rails.application.config.middleware.use(Gitlab::Jira::Middleware)
+#
+# Gitlab::Jira::Middleware is only defined on EE
+#
+# Use safe_constantize because the class may exist but has not been loaded yet
+if "Gitlab::Jira::Middleware".safe_constantize
+  Rails.application.config.middleware.use(Gitlab::Jira::Middleware)
+end