Image: modernize C++11

* use nullptr

src/Mod/Image/Gui/AppImageGuiPy.cpp

@@ -74,7 +74,7 @@ private:
 
         // Extract image into a general RGB format recognised by the ImageView class
         int format = IB_CF_RGB24;
-        unsigned char *pPixelData = NULL;
+        unsigned char *pPixelData = nullptr;
         if (imageq.isNull() == false) {
             pPixelData = new unsigned char[3 * (unsigned long)imageq.width() * (unsigned long)imageq.height()];
             unsigned char *pPix = pPixelData;