Fix http_io to make it return path is nil

What does this MR do?

If trace is in FileStorage

• Gitlab::Ci::Trace::Stream#stream.path => #{path to trace}
• Gitlab::Ci::Trace::Stream#file? => true

If trace is in ObjectStorage

• Gitlab::Ci::Trace::Stream#stream.path => nil
• Gitlab::Ci::Trace::Stream#stream.url => #{url to file in OS}
• Gitlab::Ci::Trace::Stream#file? => false

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

closes https://gitlab.com/gitlab-org/gitlab-ee/issues/5119