[I] `NULL::` can't be encode to substrait [datafusion]

[via GitHub]

discord9 opened a new issue, #15855:
URL: https://github.com/apache/datafusion/issues/15855

   ### 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:
   ```rs
   #[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_


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