Empty result iterator's hasNext() returning true

Calling hasNext() method on an empty lazy-loaded query result iterator
returns true while next() method returns null. Consequently, iterating
over such a list results in a single null element being processed.

The reason for this strange behavior is QueryResultIterator's hasNext()
relying on the existence of at least one candidate result, failing to
check if candidate's cursor contains at least one object.

This quick fix ensures that only candidates with a non-empty cursor are
added to query result candidate list.

src/java/org/datanucleus/store/mongodb/query/LazyLoadQueryResult.java

@@ -113,7 +113,10 @@ public CandidateClassResult(AbstractClassMetaData cmd, DBCursor curs, int[] fpMe
 
     public void addCandidateResult(AbstractClassMetaData cmd, DBCursor cursor, int[] fpMembers)
     {
-        candidateResults.add(new CandidateClassResult(cmd, cursor, fpMembers));
+        if (cursor.hasNext())
+        {
+            candidateResults.add(new CandidateClassResult(cmd, cursor, fpMembers));
+        }
     }
 
     /**