fix(hogql): null handling with == and != comparisons, attempt 2

[mariusandra]

Problem

Attempt nr 2 for #16216

Changes

Compared to the previous PR, this implements a nullable property on all fields (default False), which opts fields into special null handling. I found only two fields in the database that were nullable, both on the session_recordings table. All properties are still treated as nullable.

I kept the join constraint node and check, as without true type checking and propagation, we can't say if a join constraint from a subquery is nullable or not.

How did you test this code?

• bin/tests posthog/hogql
• Ran a query in metabase with the ifNull team_id check (last PR) and without it (this PR), and found this performed a lot faster
• Ran a more complex query in metabase with person joins. Verified that using isNull(argMax()) around argMax code inside those lazy tables adds no slowdown.

[Twixes]

This should be a lot better 👌