[CT-2634] [Bug] not {{ equals(actual, expected) }} isn't working properly

[dbeatty10]

Is this a new bug in dbt-core?

• I believe this is a new bug in dbt-core
• I have searched the existing issues, and I could not find an existing issue for this bug

Current Behavior

Surfaced by @lllong33 in #7670 (comment):

@sdebruyn @dbeatty10 It looks like not null is not true, can't write it like that. FYI

select version();  -- PostgreSQL 12.3 on x86_64-pc-linux-gnu

with t1 as (
	select 1 as actual, 2 as expected
	union all 
	select 1 as actual, 1 as expected
	union all 
	select 1 as actual, cast(null as int) as expected
	union all 
	select cast(null as int) as actual, cast(null as int) as expected 
)

select * from t1 
--where actual = expected or (actual is null and expected is null) -- normal: 1,1; null,null
--where actual != expected 
--	or (actual is null and expected is not null )
--	or (actual is not null and expected is null ) -- normal: 1,2; 1,null
where not (actual = expected or (actual is null and expected is null)) -- issue: actual=1, expected=null is not work 

Expected Behavior

Negation of equals should work.

Steps To Reproduce

See #7670 (comment) for a reproduction case.

Relevant log output

No response

Environment

- OS:
- Python:
- dbt:

Which database adapter are you using with dbt?

No response

Additional Context

The three-valued logic of SQL is tricky, and we should use a "fully equivalent substitute for A is not distinct from B that works in all SQL databases."

And then we should make the sure the "adapters zone" includes relevant tests.