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/itkPhaseCorrelationImageRegistrationMethod.h

@@ -345,7 +345,7 @@ class ITK_TEMPLATE_EXPORT PhaseCorrelationImageRegistrationMethod : public Proce
     }
     else
     {
-      return 0;
+      return nullptr;
     }
   }
   virtual ComplexConjugateImageType *
@@ -358,7 +358,7 @@ class ITK_TEMPLATE_EXPORT PhaseCorrelationImageRegistrationMethod : public Proce
     }
     else
     {
-      return 0;
+      return nullptr;
     }
   }