Fix non-HTML requests with vue_issues_list

What does this MR do and why?

This fixes a regression from !79408 (merged)

We still need to query the issues for the ATOM feed even if vue_issues_list is enabled

How to set up and validate locally

Comment out https://gitlab.com/gitlab-org/gitlab/blob/2ff1fbe7b48e05aeea3314c542a9adf5dd50f738/spec/spec_helper.rb#L297 so that the FF is enabled in specs.

Then run:

bin/rspec './spec/requests/projects/issues_controller_spec.rb[1:2:1:1:1:1:1:2:1]'
bin/rspec './spec/requests/groups_controller_spec.rb[1:1:1:2:1:1:1:2:1]'

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #346390 (comment 834432026)