Fix a misspelling in sync-stable-branch script

What does this MR do?

While reviewing !21647 (merged), I've noticed that the stable branch sync script has empty misspelled as empy. This MR fixes it.

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