fix: _where() renders None conditions as IS NULL instead of = NULL

col = $n bound to NULL never matches in sql, so get/get_one/exists/delete
silently missed every row filtered on a None value despite insert() writing
NULL fine. Kept in lockstep with the mysql lib's identical fix.

Signed-off-by: disqualifier <dev@disqualifier.me>

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "hatchling.build"
 
 [project]
 name = "psql"
-version = "0.1.2"
+version = "0.1.3"
 description = "async postgres wrapper over asyncpg: two-layer API (friendly verbs + raw escape hatch), fail-loud, config-free"
 requires-python = ">=3.10"
 dependencies = [