Spreadsheet: modernize C++11

* use nullptr

src/Mod/Spreadsheet/Gui/AppSpreadsheetGui.cpp

@@ -77,7 +77,7 @@ private:
     Py::Object open(const Py::Tuple& args)
     {
         char* Name;
-        const char* DocName=0;
+        const char* DocName=nullptr;
         if (!PyArg_ParseTuple(args.ptr(), "et|s","utf-8",&Name,&DocName))
             throw Py::Exception();
         std::string EncodedName = std::string(Name);
@@ -112,7 +112,7 @@ PyMOD_INIT_FUNC(SpreadsheetGui)
 {
     if (!Gui::Application::Instance) {
         PyErr_SetString(PyExc_ImportError, "Cannot load Gui module in console application.");
-        PyMOD_Return(0);
+        PyMOD_Return(nullptr);
     }
 
     // instantiating the commands