Rename ColumnModifier to SortOrder, ensure it cannot be null.
Don't use null as SortOrder.
Cleanup SortOrder enum.
Rename System.Catalog column name from column modifier to sort order.
Add SortOrder unit test.
Rename DescColumnSortOrderTest to SortOrderFTest.
Rename DescColumnSortOrderExpressionTest to SortOrderExpressionTest.
Cleanup Preconditions.checkNotNull usage for ensuring SortOrder is not null.