Fix group issues list not showing subgroup issues

What does this MR do and why?

When:

• The feature flag vue_issues_list is enabled
• A group has no issues
• A subgroup of the group has issues

Then the group issues list does not show the subgroup issues.

The expected behaviour is that it does show the subgroup issues, which this MR fixes.

Screenshots or screen recordings

before	after

How to set up and validate locally

1. In rails console enable the feature flag

   Feature.enable(:vue_issues_list)

2. Set up a group/subgroup structure like in the description then visit the group issues list

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.