NULL::<Data type> can't be encode to substrait

[discord9]

Describe the bug

as title, NULL::<Data type> can't be encode to substrait, WIP fix in #15854

To Reproduce

try run this unit test:

#[tokio::test]
    async fn fold_cast_null(){
        let state = SessionStateBuilder::default().build();
        let empty_schema = DFSchemaRef::new(DFSchema::empty());
        let field = Field::new("out", DataType::Int32, false);

        let expr = Expr::Literal(ScalarValue::Null).cast_to(&DataType::Int32, &empty_schema).unwrap();
    
        let typed_null =to_substrait_extended_expr(&[(&expr, &field)], &empty_schema, &state).unwrap();
}

and it will fail with "Null cast is invalid" or something

Expected behavior

No response

Additional context

No response