Backwards compatible active_users param

What does this MR do?

In !28016 (closed), it was discovered that the new active_users param added to the SyncSeatLinkRequestWorker in !27481 (merged) wasn't added with an optional nil value as recommended in the docs. This is recommended as existing jobs that are missing this param would fail. This MR addressed this problem by simply adding the optional value.

Follow-up issue which tracks the removal: https://gitlab.com/gitlab-org/gitlab/-/issues/212905

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team