private void copyFieldValuesThatExist(Hit filledHit, Hit hitToFill, Set<String> fieldsToFill) {
        for (String fieldToFill : fieldsToFill ) {
            if ( filledHit.getField(fieldToFill) != null ) {
                hitToFill.setField(fieldToFill, filledHit.getField(fieldToFill));
            }
        }
    }
Returns a query clone which has sourcr, offset and hits set to null. This is used by access to
the maps using the query as key to achieve lookup independent of offset/hits value
container-search/src/main/java/com/yahoo/search/searchchain/testutil/DocumentSourceSearcher.java