COMP: Remove pointer-to-bool conversions from IOFactoryRegisterManager

Fixed clang-tidy (LLVM 13.0.0) warnings from the implementation of
`IOFactoryRegisterManager` constructors, saying:

> warning: implicit conversion 'void (*)()' -> bool
> [readability-implicit-bool-conversion]

Author: N-Dekker

CMake/itkImageIOFactoryRegisterManager.h.in

@@ -26,7 +26,7 @@ class ImageIOFactoryRegisterManager
   public:
   explicit ImageIOFactoryRegisterManager(void (*list[])(void))
     {
-    for(;*list; ++list)
+    for(;*list != nullptr; ++list)
       {
       (*list)();
       }

CMake/itkMeshIOFactoryRegisterManager.h.in

@@ -26,7 +26,7 @@ class MeshIOFactoryRegisterManager
   public:
   explicit MeshIOFactoryRegisterManager(void (*list[])(void))
     {
-    for(;*list; ++list)
+    for(;*list != nullptr; ++list)
       {
       (*list)();
       }

CMake/itkTransformIOFactoryRegisterManager.h.in

@@ -26,7 +26,7 @@ class TransformIOFactoryRegisterManager
   public:
   explicit TransformIOFactoryRegisterManager(void (*list[])(void))
     {
-    for(;*list; ++list)
+    for(;*list != nullptr; ++list)
       {
       (*list)();
       }