1. Fix `order by` inability to access `select` output aliases and aggregate functions after `group by`. 2. Introduce `LUA_SQL_NULL` sentinel to preserve column ordering when `select` expressions evaluate to `nil`. Note: In Lua, setting a table key to `nil` deletes it so rows with `nil` values lose those keys and produced inconsistent insertion order across result tables. Signed-off-by: Matouš Jan Fialka <mjf@mjf.cz>