avoid situation where the linkage data could be partially calculated

[barelyknown]

The existing tests pass, though I did not take the time to create a failing test case for the situation (mostly because the easiest way would have been to test the to_one_linkage and to_many_linkage methods directly, and I didn't have time to figure out how to isolate them effectively).

This fixes the situation where the current user does not have permission to access a relationship, and the library was returning a partially complete data object for that relationship when it was polymorphic (the id would be provided, but the type would be an empty string because of the way that Relationship.type_for_source works). To keep it all simple and avoid any side effects, I'm just not returning a partial data object (where either the id or the type are nil) now.

[barelyknown]

Hold on with this. Need to manually test a bit more.

[barelyknown]

OK, I needed to guard against empty strings too. It should be good now.

[lgebhardt]

@barelyknown Thanks!