Upstream AceDB-3.0's simple-value defaults metatable uses
`function(t,k2) return k2~=nil and v or nil end` — when `v` is itself
falsy (`false`, `0`, `""`), the `and` short-circuits to that falsy
value and the trailing `or nil` then collapses it to nil, so any
`['*'] = false` (or similar) default silently reads back as nil.
Backport of https://github.com/WoWUIDev/Ace3/pull/10 (open upstream since
2023-11-04, not merged). Documented as CoA-compat patch #4 in README;
drop it when upstream finally merges.
florian.berthold