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>
Matouš Jan Fialka
f7dc242790