`==.` and `/=.` should use `IS NULL` if possible?

It's a bit of a footgun to write `table ^. SomeField ==. val Nothing`, since this expression is always equivalent to `NULL` in SQL due to trivalued logic.

We could instead check (somehow) if we're doing a comparison to a `Nothing` and change the expression to `IS NULL` or `IS NOT NULL`.

No files, tests, or entry points are named. Start by locating the implementations of ==. and /=. and the SQL expression rendering path, then determine how comparisons involving Nothing can be recognized and covered; done means producing IS NULL or IS NOT NULL where appropriate without changing other comparisons.

Written by the indexing model from the issue text.