Return type of the struct function is always a struct with an empty list of fields

[oridag]

Describe the bug

Same as title

To Reproduce

use datafusion::prelude::*;
use datafusion_expr::BuiltinScalarFunction;
​
#[tokio::main]
async fn main() -> datafusion::error::Result<()> {
 let ctx = SessionContext::new();
 ctx.register_csv("example", "tests/data/example.csv", CsvReadOptions::new()).await?;
​
 let df = ctx.table("example").await?;
​
 let f = Expr::ScalarFunction(datafusion_expr::expr::ScalarFunction {
 fun: BuiltinScalarFunction::Struct,
 args: vec![col("a"), col("b")],
 });
​
 let df2 = df.select(vec![f])?;
 df2.show().await?;
 Ok(())
}

Expected behavior

No error

Additional context

Returns the following error:

Error: ArrowError(InvalidArgumentError("column types must match schema types, expected Struct([]) but found Struct([Field { name: \"c0\", data_type: Int64, nullable: true, dict_id: 0, dict_is_ordered: false, metadata: {} }, Field { name: \"c1\", data_type: Int64, nullable: true, dict_id: 0, dict_is_ordered: false, metadata: {} }]) at column index 0"))

[jiangzhx]

@oridag thanks.
I pulled #6594 to resolve this bug.