Fix NPE when querying with prefetch

When I also want to get the referenced documents in the view query I need to set prefetch to true. If I then reference a document that doesn't exist (yet) in the DB, this causes a NPE in CBL, while CouchDB just returns "null" for the document.

This should fix that behaviour, but I'm not sure if `docContents` being null will create any problems.
Merge pull request #298 from reisub/patch-1

Fix NPE when querying with prefetch