support of empty k/v in dupsort

What's Changed

• green ci by @AskAlexSharov in #126
• gcc warnings supress by @AskAlexSharov in #125
• support empty key/values: 1. as valid dupsort key/values 2. as valid Set/SetRange/GetBoth/GetBothRange arguments. by @AskAlexSharov in #123

Full Changelog: v0.35.0...v0.36.1