For example Null can be represented as empty string

```sql
select arrayStringConcat(['x',Null], ';');

┌─arrayStringConcat(['x', Null], ';')─┐
│ x;                                  │
└─────────────────────────────────────┘
```

But mostly it for avoiding a cast in case of Nullable Strings
```
SELECT arrayStringConcat(col0, ';')
FROM
(
    SELECT [toNullable('x')] AS col0
) AS t
```

Also I think arrayStringConcat can support any types not only Strings, it allows to avoid explicit casts.

```
SELECT arrayStringConcat(col0, ';')
FROM
(
    SELECT [1, 2, 3] AS col0
) AS t
```
{
        pos = pos_;
        end = end_;
    }
    bool get(Pos & token_begin, Pos & token_end)
    {
        if (!pos)
            return false;
        token_begin = pos;
        pos = reinterpret_cast<Pos>(memchr(pos, sep, end - pos));
        if (pos)
        {
            token_end = pos;
            ++pos;
        }
        else
            token_end = end;
        return true;
    }
};
class SplitByStringImpl
{
private:
    Pos pos;
    Pos end;
    String sep;
public:
    static constexpr auto name = "splitByString";
    static String getName() { return name; }