Expand Step create nullmap used wrong datatype ColumnInt8


The code in ExpandStep use ColumnInt8 for nullmap, will occurred `Bad cast from type DB::ColumnVector<signed char> to DB::ColumnVector<char8_t>`
```c++
auto null_map = DB::ColumnInt8::create(rows, 1);
auto col = DB::ColumnNullable::create(original_col, std::move(null_map));
cols.push_back(std::move(col));
```

Use `ColumnUInt8` instead.
