Added Nulls getter to manipulate internal nulls on ColumnNullable

[dgrr]

Hello,

This commit will allow the programmer to access the nulls_ column.
In my use case, I need to access the nulls too. This is a similar approach:

auto col = columns[0]->As<ColumnNullable>();
col->Nested()->As<ColumnString>()->Append(this->value);
col->Nulls()->As<ColumnUInt8>()->Append(!this->is_value_ok());

Thanks.

[artpaul]

Thanks!