Added equals macro that handles null value comparison

[adrianburusdbt]

resolves #

• [CT-2704] [Bug] When any of the unique keys is null incremental model duplicates data dbt-core#7873
• [CT-2563] [Bug] Incremental updates using unique_key result in duplicates if fields in the unique_key are null #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.

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

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[VersusFacit]

thanks @mikealfare for doing the bulk of CR here.