Add not_empty_string test

[epapineau]

This is a:

• documentation update
• bug fix with no breaking changes
• new functionality
• a breaking change

All pull requests from community contributors should target the main branch (default).

Description & motivation

Resolves #632

Checklist

• I have verified that these changes work locally on the following warehouses (Note: it's okay if you do not have access to all warehouses, this helps us understand what has been covered)
  • BigQuery
  • Postgres
  • Redshift
  • Snowflake
• I followed guidelines to ensure that my changes will work on "non-core" adapters by:
  • dispatching any new macro(s) so non-core adapters can also use them (e.g. the star() source)
  • using the limit_zero() macro in place of the literal string: limit 0
  • using dbt_utils.type_* macros instead of explicit datatypes (e.g. dbt_utils.type_timestamp() instead of TIMESTAMP
• I have updated the README.md (if applicable)
• I have added tests & descriptions to my models (and macros if applicable)
• I have added an entry to CHANGELOG.md

[epapineau]

We discussed on the original issue also adding a not_empty_array test, but I need to put more thought into what that would look like from a testing perspective (see core issue 2365). I wanted to get this out for review now though.

[joellabes]

I like where this is going! Thank you for your patience 🙏 a couple of comments/questions where I'm not sure that I'm understanding it correctly, but otherwise let's get this merged

[joellabes]

Should these be greater than signs?

[joellabes]

No, they shouldn't! This is a very clever way to force the tests to check the negative and positive case.

I really like this 🧠

[epapineau]

All credit goes to @dbeatty10 on this approach, but it feels good knowing that we're testing both the pass and fails.

[joellabes]

@epapineau this looks great! just checking if you meant to change the postgres env vars?