Thank you!
Overriding SortExpression solves our problem!
public class DerivedSortExpression : SortExpression
{
public override string ToQueryText(ref int uniqueMarker, bool aliasesForExpressionsAggregates)
{
return base.ToQueryText(ref uniqueMarker, aliasesForExpressionsAggregates).Replace("DESC","DESC NULLS LAST").Replace("ASC","ASC NULLS FIRST");
}
}