ceil and floor panic on a decimal scalar when number_rows is 0 #26950

Describe the bug

ceil and floor turn a decimal scalar argument into an array of number_rows rows and then read row 0 of the result. With number_rows = 0 that array is empty, and the read panics:

Trying to access an element at index 0 from a PrimitiveArray of length 0

The comment above the scalar path says decimals are converted "to array of size 1", but the conversion uses args.number_rows:

• ceil: comment, conversion, read of row 0
• floor: comment, conversion, read of row 0

Float scalars return before this point, so only decimals are affected.

To Reproduce

On main at 926901f:

use std::sync::Arc;

use arrow::datatypes::Field;
use datafusion_common::ScalarValue;
use datafusion_common::config::ConfigOptions;
use datafusion_expr::{ColumnarValue, ReturnFieldArgs, ScalarFunctionArgs};
use datafusion_functions::math::ceil;

let udf = ceil();
let arg = ScalarValue::Decimal32(Some(150), 9, 2);
let arg_fields = vec![Arc::new(Field::new("a", arg.data_type(), true))];
let return_field = udf
    .return_field_from_args(ReturnFieldArgs {
        arg_fields: &arg_fields,
        scalar_arguments: &[Some(&arg)],
    })
    .unwrap();
let result = udf.invoke_with_args(ScalarFunctionArgs {
    args: vec![ColumnarValue::Scalar(arg)],
    arg_fields,
    number_rows: 0, // with 1: Ok(Scalar(Decimal32(2, 8, 0)))
    return_field,
    config_options: Arc::new(ConfigOptions::default()),
});
// panics: Trying to access an element at index 0 from a PrimitiveArray of length 0

floor panics the same way. With number_rows: 1, ceil returns Decimal32(2, 8, 0) and floor returns Decimal32(1, 8, 0).

Expected behavior

The same scalar as with number_rows: 1, as the comment describes. A scalar result doesn't depend on the number of rows.

Additional context

Found by property-based tests that call each function directly with generated arguments, including scalar arguments with zero rows.

Willingness to contribute

I cannot contribute a fix for this bug at this time