Allow empty future subscriptions on seat link sync

What does this MR do and why?

Please refer to the Summary of https://gitlab.com/gitlab-org/gitlab/-/issues/353283 for all the specifics regarding this change.

Screenshots or screen recordings

Does not apply.

How to set up and validate locally

Validation steps added at the issue (https://gitlab.com/gitlab-org/gitlab/-/issues/353283) level here

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.