Fixed storing and loading nulls and booleans in Redis

See #1311 

- [ ] Fix `LuaScriptBuilder` to use `HEXISTS` in case of `null` values instead of comparison.
- [x] Fix `LuaScriptBuilder` to cast booleans to 0 or 1 before using these in conditions.
There's no way to store `null` in a Redis hash key so we're not storing a key at all in this case. It results in a missing key when such a hash is read.