public void unifyNull() {
        class FindGroup implements ObjectPredicate {

            @Override
            public boolean check(Object obj) {
                return obj instanceof Group;
            }
        }
        class UnifyNullGroupId implements ObjectOperation {

            @Override
            public void execute(Object obj) {
                Group group = (Group)obj;
                ResultNode id = group.getId();
                if (id instanceof BucketResultNode && ((BucketResultNode)id).empty()) {
                    group.setId(new NullResultNode());
                }
            }
        }
        selectMembers(new FindGroup(), new UnifyNullGroupId());
    }
Returns whether ordering will need results collected in children.
In that case we will probably just do a single pass.
searchlib/src/main/java/com/yahoo/searchlib/aggregation/Grouping.java