Fix SectionService empty list validation

[NikosPoly]

Currently SectionService rejects empty lists [] for bookIds and employeeIds
during PATCH, making it impossible to intentionally clear a section's books or employees.

The intended behavior should be:

• null — don't touch the list
• [] — clear the list
• ["id1", "id2"] — replace the list

However the correct approach for clearing a list via PATCH needs further consideration
before implementation.

Tasks

• Decide on the correct approach for clearing a list via PATCH
• Implement the chosen approach in SectionService
• Verify that null vs empty list behavior works correctly after the fix