Write test for nulled fragment data

[rexxars]

I'm having a hard time figuring out the flow and how this bug occurs. Basically it has to do with references that can't be resolved (0, for instance). They are returned from the fetcher as null and something in the resolve-fragment code tries to map over the values and extract the IDs from it. That obviously fails when encountering a null.

I hotfixed it with d566b43, but someone(tm) should add a test to ensure there are no regressions later.