COMP: Fix modernize-use-nullptr warning in vtkSMPToolsAPI::ClassFinalize
This fixes the following warning introduced in e5fa6f9c (BUG: Resolve crash by fixing initialization of backend-specific statics) through !10751 (merged)
Common/Core/SMP/Common/vtkSMPToolsAPI.cxx:76:37: warning: use nullptr [modernize-use-nullptr]
vtkSMPToolsAPIInstanceAsPointer = 0;
^
nullptr