message: docs: remove NULL strings from highgui.hpp
diff: @@ -532,7 +532,7 @@ displayed in the specified window winname.
  @note
  -[__Qt Backend Only__] winname can be empty (or NULL) if the trackbar should be attached to the
  +[__Qt Backend Only__] winname can be empty if the trackbar should be attached to the
  control panel.
  Clicking the label of each trackbar enables editing the trackbar values manually.
  @@ -560,7 +560,7 @@ The function returns the current position of the specified trackbar.
  @note
  -[__Qt Backend Only__] winname can be empty (or NULL) if the trackbar is attached to the control
  +[__Qt Backend Only__] winname can be empty if the trackbar is attached to the control
  panel.
  @param trackbarname Name of the trackbar.
  @@ -574,7 +574,7 @@ The function sets the position of the specified trackbar in the specified window
  @note
  -[__Qt Backend Only__] winname can be empty (or NULL) if the trackbar is attached to the control
  +[__Qt Backend Only__] winname can be empty if the trackbar is attached to the control
  panel.
  @param trackbarname Name of the trackbar.
  @@ -589,7 +589,7 @@ The function sets the maximum position of the specified trackbar in the specifie
  @note
  -[__Qt Backend Only__] winname can be empty (or NULL) if the trackbar is attached to the control
  +[__Qt Backend Only__] winname can be empty if the trackbar is attached to the control
  panel.
  @param trackbarname Name of the trackbar.
  @@ -604,7 +604,7 @@ The function sets the minimum position of the specified trackbar in the specifie
  @note
  -[__Qt Backend Only__] winname can be empty (or NULL) if the trackbar is attached to the control
  +[__Qt Backend Only__] winname can be empty if the trackbar is attached to the control
  panel.
  @param trackbarname Name of the trackbar.
  @@ -811,7 +811,7 @@ QT_NEW_BUTTONBAR flag is added to the type.
  See below various examples of the cv::createButton function call: :
  @code
  - createButton(NULL,callbackButton);//create a push button "button 0", that will call callbackButton.
  + createButton("",callbackButton);//create a push button "button 0", that will call callbackButton.
  createButton("button2",callbackButton,NULL,QT_CHECKBOX,0);
  createButton("button3",callbackButton,&value);
  createButton("button5",callbackButton1,NULL,QT_RADIOBOX);
  