Image: modernize C++11

* remove redundant void-arg
* use nullptr
* replace deprecated headers

src/Mod/Image/Gui/AppImageGui.cpp

@@ -23,7 +23,7 @@
 #include "ViewProviderImagePlane.h"
 
 // use a different name to CreateCommand()
-void CreateImageCommands(void);
+void CreateImageCommands();
 
 void loadImageResource()
 {
@@ -42,7 +42,7 @@ PyMOD_INIT_FUNC(ImageGui)
 {
     if (!Gui::Application::Instance) {
         PyErr_SetString(PyExc_ImportError, "Cannot load Gui module in console application.");
-        PyMOD_Return(0);
+        PyMOD_Return(nullptr);
     }
 
     PyObject* mod = ImageGui::initModule();