DerivedDataEngine always (re)creates multi-valued hippo:related property even if empty

There is a minor bug in the DerivedDataEngine when updating the (multi-valued) derived hippo:related property: even if there are now related references it still will (re)create that property.
With as result that you'll always will have this property, removing it has no persisted effect either.
This bug does no harm, but nonetheless annoying, while fixing it is trivial.