CAY-1905 Multi-step prefetching NPE : 1..N..1 with absent N and root with no qualifier

fix suggested by Mikaël Cluseau (we don't have a test for it)

git-svn-id: https://svn.apache.org/repos/asf/cayenne/main/branches/STABLE-3.1@1578749 13f79535-47bb-0310-9956-ffa450edef68