Fix not equals comparison to be null-safe for adapters/utils tests

[dbeatty10]

resolves #7778

follow-up on #7672

Description

The bug report describes how functional tests in adapters/utils are not currently null-safe (specifically for "not equals" comparisons).

Main things

This pull request does two main things:

1. Implements a "SQL standard conforming alternative" to A is not distinct from B.
2. Implements comprehensive 🤞 tests inspired by the Snowflake docs and demo'd here.

Ancillary things

This PR needed some ancillary things as well that arose with the improved equals comparisons:

• Fix tests for concat and hash by using empty strings ('') instead of null

See a combination of three-valued logic of SQL and #4725 for context why these changes were necessary and done this particular way.

Future-facing opportunities

The code in this PR would make the implementation of #6997 as easy as possible:

1. Move the definition of the equals macro from tests/adapter/dbt/tests/adapter/utils/base_utils.py to:

   core/dbt/include/global_project/macros/utils/equals.sql

   {% macro equals(expr1, expr2) -%}
     {{ return(adapter.dispatch('equals', 'dbt')(expr1, expr2)) }}
   {%- endmacro %}
   
   {% macro default__equals(expr1, expr2) -%}
   case when (({{ expr1 }} = {{ expr2 }}) or ({{ expr1 }} is null and {{ expr2 }} is null))
       then 0
       else 1
   end = 0
   {%- endmacro %}
   

2. Remove any other code that is no longer necessary

3. Adopt this test within each adapter

4. Resolve this issue: Cross-database null-safe equals comparison via equals docs.getdbt.com#3469

5. Add the new macro to the migration guide for adapter maintainers

Checklist

• I have read the contributing guide and understand what's expected of me
• I have signed the CLA
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests 👍
• Updates to docs are not relevant
• I have run changie new to create a changelog entry

[github-actions]

Thank you for your pull request! We could not find a changelog entry for this change. For details on how to document a change, see the contributing guide.

[dbeatty10]

Good news: the functional test implemented in 617cf1e is now catching the exact problem reported by @lllong33 in #7670 (comment).

https://github.com/dbt-labs/dbt-core/actions/runs/5170455117/jobs/9313364211?pr=7776

[VersusFacit]

Glad to know there's even more ways to test equality than I thought. When we do adapter zone tests, we'll have to watch our backs on the fact that things like distinct from may not exist on all dbms.

Regardless, approved (assuming your responses are "yep" to comments as I'm expecting them to be)