Merge branch 'fix/mirror-update-error' into 'master'

Fix empty import URL errors

CE version of https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/780

See merge request !6693

app/models/project.rb

@@ -490,7 +490,7 @@ class Project < ActiveRecord::Base
   end
 
   def import_url
-    if import_data && super
+    if import_data && super.present?
       import_url = Gitlab::UrlSanitizer.new(super, credentials: import_data.credentials)
       import_url.full_url
     else