COMP: Use nullptr instead of 0 or NULL

The check converts the usage of null pointer constants (eg. NULL, 0) to
use the new C++11 nullptr keyword.

SRCDIR= #My local SRC
BLDDIR= #My local BLD

cd
run-clang-tidy.py -extra-arg=-D__clang__ -checks=-*,modernize-use-nullptr  -header-filter=.* -fix

include/itkBSplineApproximationGradientImageFilter.hxx

@@ -53,7 +53,7 @@ BSplineApproximationGradientImageFilter<TInputImage, TOutputValueType>::Generate
   Superclass::GenerateInputRequestedRegion();
 
   typename InputImageType::Pointer input = const_cast<InputImageType *>(this->GetInput());
-  if (input.GetPointer() == NULL)
+  if (input.GetPointer() == nullptr)
   {
     return;
   }
@@ -76,7 +76,7 @@ BSplineApproximationGradientImageFilter<TInputImage, TOutputValueType>::Generate
 {
   typename InputImageType::ConstPointer input = this->GetInput();
 
-  if (input.GetPointer() == NULL)
+  if (input.GetPointer() == nullptr)
   {
     return;
   }