vtkSphere : passing a null parameter in ComputeBoundingSphere from Python

[dgobbi (David Gobbi)]

Yes, overloads are preferred over default arguments.

https://docs.vtk.org/en/latest/developers_guide/coding_conventions.html

35. Prefer overloading functions to default arguments.
    Rationale: Default function arguments in C++ are a tempting way to add an argument to a function while maintaining easy backwards compatibility. However, if you later want to add another argument to the list in a way that preserves backwards compatibility, it, too, must be a default argument. To supply the second of these arguments in a call forces you to also supply the first argument, even if it is the default value. As a result, this is not a clean way to add a argument to a function. Insetad, function overloading should be preferred.