[Rails5] NoMethodError: undefined method `join' for "":String

Note: add rails5 to a branch name to trigger rails5 specific CI jobs.

https://gitlab.com/gitlab-org/gitlab-ee/-/jobs/77073866

  18) Oauth::ApplicationsController project members POST #create logs the audit event
      Failure/Error: params[:doorkeeper_application][:scopes] = scopes.join(' ')

      NoMethodError:
        undefined method `join' for "":String
      # ./app/controllers/concerns/oauth_applications.rb:12:in `prepare_scopes'
      # ./ee/spec/controllers/oauth/applications_controller_spec.rb:20:in `block (5 levels) in <top (required)>'
      # ./ee/spec/controllers/oauth/applications_controller_spec.rb:20:in `block (4 levels) in <top (required)>'