[Bug] Unit test fixture null values cause typing issues

[VersusFacit]

Is this a new bug in dbt-core?

• I believe this is a new bug in dbt-core
• I have searched the existing issues, and I could not find an existing issue for this bug

Current Behavior

Unit test fixtures are parsed correctly, but they leave typing up to the target database. This will result in unhandled database failures when there is no one row Null free (this handles indirectly the all values in a column Null failure case). The database won't be able to determine which type to use and default to something like VARCHAR(1) which may not be what the source table expects -- this leads to a type failure.

This change is motivated by a redshift change but could affect other adapters some day.

Expected Behavior

Let's sort this to give fixtures the best chance to succeed or in unwinnable situations, error out with a known exception.

Steps To Reproduce

See reprex here

Relevant log output

No response

Environment

- OS:
- Python:
- dbt:

Which database adapter are you using with dbt?

redshift

Additional Context

See redshift PR here