Always display Labels section in issuable sidebar, even when the project has no labels

Originally opened at !13859 (closed) by @brancho.

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What does this MR do?

Fixes the issue - https://gitlab.com/gitlab-org/gitlab-ce/issues/23472

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

Please check the screenshot to ensure that this satisfies UX criteria

Why was this MR needed?

https://gitlab.com/gitlab-org/gitlab-ce/issues/23472

Screenshots

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #23472 (closed).