MINOR: fix concat_ws corner bug

[WinkerDu]

Which issue does this PR close?

Closes #2100.

Rationale for this change

Function concat_ws is not ignoring NULL in last argument and seperator text is added at the end of the result.
To Reproduce

SELECT concat_ws('|', 'a', NULL);
-- Result: a|

Expected behavior

SELECT concat_ws('|', 'a', NULL);
-- Result: a

What changes are included in this PR?

skip adding separator when NULL in last argument.

Are there any user-facing changes?

No.

[mateuszkj]

Now it's failing with two NULLs at last arguments.

To Reproduce

SELECT concat_ws('|','a',NULL, NULL);
-- Result: a|

Expected behavior

SELECT concat_ws('|','a',NULL, NULL);
-- Result: a

I would suggest to append separator before pushing value. E.g.

let mut owned_string: String = "".to_owned();
let mut first = true;
for arg_index in 1..args.len() {
    let arg = &args[arg_index];
    if !arg.is_null(index) {
        // if not first push separator
        if !first {
            owned_string.push_str(sep);
            first = false;
        }
        owned_string.push_str(arg.value(index));
    }
}

[doki23]

How about

  let mut owned_string = Vec::with_capacity(args.len()-1);
  for arg_index in 1..args.len() {
      let arg = &args[arg_index];
      if !arg.is_null(index) {
          owned_string.push(arg.value(index));
      }
  }
  owned_string.join(sep)

[Dandandan]

How about

  let mut owned_string = Vec::with_capacity(args.len()-1);
  for arg_index in 1..args.len() {
      let arg = &args[arg_index];
      if !arg.is_null(index) {
          owned_string.push(arg.value(index));
      }
  }
  owned_string.join(sep)

Could also use an iterator + flat_map + collect (into Vec<String>) for a rusty version.

[WinkerDu]

@doki23 @Dandandan thanks for the advice, I'd try it

[WinkerDu]

@mateuszkj @doki23 @Dandandan PTAL, thanks

[Dandandan]

Suggested change

	if !arg.is_null(index) {
	vec![arg.value(index)]
	} else {
	vec![]
	if !arg.is_null(index) {
	Some(arg.value(index))
	} else {
	None
	}

[WinkerDu]

@Dandandan @houqp @alamb PTAL

[doki23]

LGTM, thanks @WinkerDu