Method hasNative

Fleece/Integration/MValue.hh:85–85

        bool isEmpty() const FLPURE        {return !_value && !_native;}
        bool isMutated() const FLPURE      {return !_value;}
        bool hasNative() const FLPURE      {return _native != nullptr;}

        Native asNative(const MCollection<Native> *parent) const {
            if (_native || !_value) {