Handle nullable columns for databases using pointers

[patrickpichler]

Some databases (e.g. clickhouse) is using pointer values to represent nullable values when querying from the database. The current logic to transform values to strings when querying data, does not handle this correctly. It will simply transform the pointer to a string, which results in the pointer address being printed.

To handle this, the value reported from the sql driver will now tried to be extracted, if it is a pointer. This means the value of the pointer will then be transformed to a string, instead of the pointer directly.

Additionally, some cases where this was causing issues in the clickhouse implementation have been fixed.

Fixes #151