Add macro to allow adapter-specific choice of unspecified value

[steffen030]

The current implementation of unit tests renders null values for unspecified columns. This however is problematic for databases like Clickhouse which know not-nullable datatypes. For these cases unit tests are currently not working.

This PR allows database adapters to override the unspecified column value.

It closes dbt-labs/dbt-core#10447

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[steffen030]

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In order for us to review and merge your code, please submit the Individual Contributor License Agreement form attached above above. If you have questions about the CLA, or if you believe you've received this message in error, please reach out through a comment on this PR.

CLA has not been signed by users: @steffen030

Should be signed now.