Rails5: fix artifacts controller download spec

What does this MR do?

Rails5 has params[:file_type] as '' if file_type is included as nil in the spec request params. This is not what happens on a real request if no file_type is send from the browser, the parameter will simply not exist. I changed the spec to send no file_type param, instead of a param that is nil.

Fixes spec/controllers/projects/artifacts_controller_spec.rb:28, See https://gitlab.com/gitlab-org/gitlab-ce/-/jobs/104515016

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides