Explicitly set to nil when artifacts don't exist:

Feedback from:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4964#note_12867273

app/models/ci/build.rb

@@ -384,6 +384,8 @@ module Ci
     def update_artifacts_size
       self.artifacts_size = if artifacts_file.exists?
                               artifacts_file.size
+                            else
+                              nil
                             end
     end