docs: remove NULL strings from highgui.hpp

modules/highgui/include/opencv2/highgui.hpp

@@ -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);