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 || pos > end)
            return false;
        if (!re->match(pos, end - pos, matches) || !matches[0].length)
            return false;
        if (matches[capture].offset == std::string::npos)
        {
            
            token_begin = pos;
            token_end = pos;
        }
        else
        {
            token_begin = pos + matches[capture].offset;
            token_end = token_begin + matches[capture].length;
        }
        pos += matches[0].offset + matches[0].length;
        return true;
    }
};
template <typename Generator>
class FunctionTokens : public IFunction