fix syntax error in pod documentation for find_or_new
Stop adding empty caches to resultsets chained off prefetched objects

Prior to ce55688, the related_rs cache loading mechanism only conditionally
loaded if the base rs cache was defined. After this was removed an empty
arrayref (meaning "no related results") could potentially be set as the cache.
This commit ensures the cache is only set when there are actual cache results.
Do not invoke the pk accessor in filter deflate