message: core(MatExpr): fix warning in case of e.s == (0, 0, 0, 0)
diff: @@ -1329,7 +1329,7 @@ void MatOp_AddEx::assign(const MatExpr& e, Mat& m, int _type) const
  }
  else if( e.s.isReal() && (dst.data != m.data || fabs(e.alpha) != 1))
  {
  - if (e.a.channels() > 1)
  + if (e.a.channels() > 1 && e.s[0] != 0.0)
  CV_LOG_ONCE_WARNING(NULL, "OpenCV/MatExpr: processing of multi-channel arrays might be changed in the future: "
  "https://github.com/opencv/opencv/issues/16739");
  e.a.convertTo(m, _type, e.alpha, e.s[0]);
  