Added equals macro that handles null value comparison

[adrianburusdbt]

resolves #
dbt-labs/dbt-core#7873
#159

Problem

Null comparison was not handled properly in the base adapter. While some value comparisons were missing this case altogether, others were fixed but not in a repeatable, organized way.

Solution

Based loosely on #110, but ultimately it's following the suggestions made here: dbt-labs/dbt-core#6997 (comment)
Extracted the logic that was in the test fixture into an 'equals.sql' macro that can be reused and replaced all other usages with this utility macro.

Integration tests with the proposed changes:
https://github.com/dbt-labs/dbt-postgres/actions/runs/12412018685

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development, and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc.) or this PR has already received feedback and approval from Product or DX

[VersusFacit]

LGTM.

✅ redshift: https://github.com/dbt-labs/dbt-redshift/actions/runs/12436459830
✅ snowflake: https://github.com/dbt-labs/dbt-snowflake/actions/runs/12436503051
✅ bigquery: https://github.com/dbt-labs/dbt-bigquery/actions/runs/12436511956
✅ spark: https://github.com/dbt-labs/dbt-spark/actions/runs/12459732059/job/34777655655

I'm waiting on a reply to Mike on why Spark is failing and if it's actually a concern here or not (I doubt it is)