LOGC-41: Add support for nullable fields in ClickHouse

[dvasilas]

Align ClickHouse schema used in tests and add support for NULL values.

The problem this solves

From AWS documentation:

Any field can be set to - to indicate that the data was unknown or unavailable, or that the field was not applicable to this request.

Bucket logging did not handle this correctly:
When a field was omitted from server-access.log JSON, and FluentBit did not insert a value for it, ClickHouse used a default value (0 for number data types, "" for string, etc)

For example, for a list object operation, object size does not apply and is omitted from the log.
But the final log had object size: 0 instead of -.