Fixed link to parent WP in the WP table

https://www.openproject.org/work_packages/14707
@mtakac you're right if you assume that ``workPackageColumnDirective`` always gets a work package object and, based on this object, has to decide which property to display (and how). But what if ``workPackageColumnDirective`` gets only a (property) value instead of a work package object? For instance, let's say you just pass an id to the directive. Whether this id is a user, a work package, or something completely different then depends on the value of ``model_type``. Whether the value represents a parent work package or another work package, then, doesn't matter for the directive anymore.