datafusion-proto does not support untyped null scalar values

[andygrove]

Describe the bug
datafusion-proto does not support untyped null scalar values

To Reproduce

  #[test]
  fn roundtrip_null_literal() {
      let test_expr = Expr::Literal(ScalarValue::Null);

      let ctx = SessionContext::new();
      roundtrip_expr_test(test_expr, ctx);
  }

Expected behavior
Should work

Additional context
None