WIP: fix clang -Wtautological-pointer-compare warnings

Warning was:

```
/Modules/Registration/GPUPDEDeformable/include/itkGPUPDEDeformableRegistrationFilter.hxx:102:7: warning: comparison of array 'this->m_GPUSmoothingKernels' not equal to a null pointer is always true [-Wtautological-pointer-compare]
  if (m_GPUSmoothingKernels != nullptr)
      ^~~~~~~~~~~~~~~~~~~~~    ~~~~~~~
```

Modules/Registration/GPUPDEDeformable/include/itkGPUPDEDeformableRegistrationFilter.hxx

@@ -90,7 +90,6 @@ GPUPDEDeformableRegistrationFilter<TFixedImage, TMovingImage, TDisplacementField
   }
 
   os << indent << "SmoothingKernels: ";
-  if (m_SmoothingKernels != nullptr)
   {
     for (unsigned d = 0; d < ImageDimension; ++d)
     {
@@ -99,7 +98,6 @@ GPUPDEDeformableRegistrationFilter<TFixedImage, TMovingImage, TDisplacementField
   }
 
   os << indent << "GPUSmoothingKernels: ";
-  if (m_GPUSmoothingKernels != nullptr)
   {
     for (unsigned d = 0; d < ImageDimension; ++d)
     {
@@ -114,7 +112,6 @@ GPUPDEDeformableRegistrationFilter<TFixedImage, TMovingImage, TDisplacementField
   }
 
   os << indent << "UpdateFieldSmoothingKernels: ";
-  if (m_UpdateFieldSmoothingKernels != nullptr)
   {
     for (unsigned d = 0; d < ImageDimension; ++d)
     {
@@ -123,7 +120,6 @@ GPUPDEDeformableRegistrationFilter<TFixedImage, TMovingImage, TDisplacementField
   }
 
   os << indent << "UpdateFieldGPUSmoothingKernels: ";
-  if (m_UpdateFieldGPUSmoothingKernels != nullptr)
   {
     for (unsigned d = 0; d < ImageDimension; ++d)
     {