From QueryResponse: a null cookie should be returned "if paged results were not requested, there are no more pages to be returned, or cookies are not supported for this query." Currently, in JDBCRepoService, line 612, a null cookie is returned only if the returned results are less than the request page size. Thus, if a QueryRequest with a page size of 13 is directed against a jdbc repo with only 13 records, a non-null paging cookie will be returned. This is a violation of the 'null cookie if no more pages will be returned' semantics.
Note that the DJ repo will implement these semantics as of