Method simplify

datafusion/functions/src/string/concat_ws.rs:385–394

Simply the `concat_ws` function by 1. folding to `null` if the delimiter is null 2. filtering out `null` arguments 3. using `concat` to replace `concat_ws` if the delimiter is an empty string 4. concatenating contiguous literals if the delimiter is a literal.

(
        &self,
        args: Vec<Expr>,
        _info: &SimplifyContext,
    )

    /// 3. using `concat` to replace `concat_ws` if the delimiter is an empty string
    /// 4. concatenating contiguous literals if the delimiter is a literal.
    fn simplify(
        &self,
        args: Vec<Expr>,
        _info: &SimplifyContext,
    ) -> Result<ExprSimplifyResult> {
        match &args[..] {
            [delimiter, vals @ ..] => simplify_concat_ws(delimiter, vals),
            _ => Ok(ExprSimplifyResult::Original(args)),
        }
    }

    fn documentation(&self) -> Option<&Documentation> {
        self.doc()