 SortOrder is similar to (or the same as) the SortOrder already defined
     * for Grid.
     * 
     * The default implementation of this method returns an empty array, which
     * means that the property will not be included in
     * getSortableContainerPropertyIds(). Attempting to sort by that column
     * throws UnsupportedOperationException.
     * 
     * Returning null is not allowed.
 SortOrder is similar to (or the same as) the SortOrder already defined
     * for Grid.
     * 
     * The default implementation of this method returns an empty array, which
     * means that the property will not be included in
     * getSortableContainerPropertyIds(). Attempting to sort by that column
     * throws UnsupportedOperationException.
     * 
     * Returning null is not allowed. | PropertyValueGenerator.java