message: add empty implementation
diff: @@ -284,7 +284,10 @@ namespace cv { namespace gapi { namespace own {
  The method returns true if Mat::total() is 0 or if Mat::data is NULL. Because of pop_back() and
  resize() methods `M.total() == 0` does not imply that `M.data == NULL`.
  */
  - bool empty() const;
  + bool empty() const
  + {
  + return data == 0 || total() == 0 || dims.empty();
  + }
  /** @brief Returns the total number of array elements.
  