Merge branch 'extract-closing-issues' into 'master'

Fix error when viewing merge request with a commit that includes "Closes #<issue id>".

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/488 was giving a 500 error, and the logs showed this was the error.

I'm not yet sure why project would be `nil` in this case, but that's another issue.

cc @rspeicher

See merge request !2

CHANGELOG-EE

+v 7.12 (Unreleased)
+  - Fix error when viewing merge request with a commit that includes "Closes #<issue id>".
+
 v 7.11.2
   - Fixed license upload and verification mechanism
 

lib/gitlab/reference_extractor.rb

@@ -23,7 +23,7 @@ module Gitlab
     end
 
     def issues
-      if project.default_issues_tracker?
+      if project.nil? || project.default_issues_tracker?
         result = pipeline_result(:issue)
       elsif project.jira_tracker?
         result = pipeline_result(:external_issue)