Fix tasklist for epics

What does this MR do?

Fix tasklist for epics. Previously, you would get an error message when checking the tasklist

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

Shouldn't be

Why was this MR needed?

regression

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Backport to CE https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15556
• Review
  • Has been reviewed by Frontend

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/4021