QSafeString::set needs nullptr check

void QSafeString::set(const qchar * const strArg, const size_t lengthArg) function requires nullptr check for the strArg. Currently std::copy will be called for strArg without nullptr check and that can lead to unexpected behavior.

Also, lengthArg should be checked for zero, and do just a reset to the m_data instead of set if the length is 0.