For csv format of unit testing, empty strings ("") and "null" both convert to SQL NULL

[FishtownBuildBot]

PR that needs documentation

dbt-labs/dbt-core#9881

🌶️ Hot take, we might consider closing this issue as just a special case of #4867 (which we should resolve!).

Page requiring updates

Not sure the best page(s). Likely one or more of the following:

• Unit tests
• Unit testing reference page
• Supported data formats for mock data
• Unit testing versioned models
• Unit test inputs
• Unit test overrides
• Platform-specific data types

Part(s) of the page to update

The csv format now works exactly the same as dbt seeds (which are only defined using CSV), so maybe we don't actually need to make any changes as a result of this PR!

More detail

We'd just need to check if there's anywhere in our unit testing docs that we mention blank / empty string values ("").

Overall, we should really just implement #4867 and make sure it covers all of these cases:

• null values in csv format for unit tests
• null values in dict format for unit tests
• null values in sql format for unit tests
• null values in CSV seeds
• empty string values in csv format for unit tests
• empty string values in dict format for unit tests
• empty string values in sql format for unit tests
• empty string values in CSV seeds