Ensure vtkPythonMap is created before using it in AddClassToMap

@dgobbi when working on a VTK module with Python wrapping (quite similar to the VTK Example Wrapping Module) I tried to install it and use it in my python shell. At first, I had segfault due to vtkPythonMap being null. I have ensured it is created using the vtkPythonUtilCreateIfNeeded method and now everything work as expected. Why do you think about this change ?

@ben.boeckel FYI.