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What are you changing in this pull request and why?

The current code example does not always work and may raise an error like this:

14:01:53  None is not of type 'integer'
14:01:53  
14:01:53  Failed validating 'type' in schema['properties']['failures']:
14:01:53      {'type': 'integer'}
14:01:53  
14:01:53  On instance['failures']:
14:01:53      None

As described in dbt-labs/dbt-core#4248, it's known that:

• tests whose fail_calc is a sum() function raises an error when the test produces no rows at all

Well, that explains why the code example doesn't work!

Jerco described two options for dealing with this:

1. rewrite the test query, to ensure it always returns at least one row
2. rewrite the fail_calc, to handle the case in which no rows are returned

This PR adopts the 2nd option and updates the code example to handle the case in which no rows are returned.

Checklist

• Review the Content style guide so my content adheres to these guidelines.
• I've verified that the updated code example works