COMP: 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)
      ^~~~~~~~~~~~~~~~~~~~~    ~~~~~~~
```

Indeed these can never be null.

Modules/Registration/GPUPDEDeformable/include/itkGPUPDEDeformableRegistrationFilter.hxx

@@ -90,21 +90,15 @@ GPUPDEDeformableRegistrationFilter<TFixedImage, TMovingImage, TDisplacementField
  }
 
  os << indent << "SmoothingKernels: ";
- if (m_SmoothingKernels != nullptr)
+ for (unsigned d = 0; d < ImageDimension; ++d)
  {
- for (unsigned d = 0; d < ImageDimension; ++d)
- {
- os << indent.GetNextIndent() << m_SmoothingKernels[d] << std::endl;
- }
+ os << indent.GetNextIndent() << m_SmoothingKernels[d] << std::endl;
  }
 
  os << indent << "GPUSmoothingKernels: ";
- if (m_GPUSmoothingKernels != nullptr)
+ for (unsigned d = 0; d < ImageDimension; ++d)
  {
- for (unsigned d = 0; d < ImageDimension; ++d)
- {
- os << indent.GetNextIndent() << m_GPUSmoothingKernels[d] << std::endl;
- }
+ os << indent.GetNextIndent() << m_GPUSmoothingKernels[d] << std::endl;
  }
 
  os << indent << "UpdateFieldSmoothingKernelSizes: ";
@@ -114,21 +108,15 @@ GPUPDEDeformableRegistrationFilter<TFixedImage, TMovingImage, TDisplacementField
  }
 
  os << indent << "UpdateFieldSmoothingKernels: ";
- if (m_UpdateFieldSmoothingKernels != nullptr)
+ for (unsigned d = 0; d < ImageDimension; ++d)
  {
- for (unsigned d = 0; d < ImageDimension; ++d)
- {
- os << indent.GetNextIndent() << m_UpdateFieldSmoothingKernels[d] << std::endl;
- }
+ os << indent.GetNextIndent() << m_UpdateFieldSmoothingKernels[d] << std::endl;
  }
 
  os << indent << "UpdateFieldGPUSmoothingKernels: ";
- if (m_UpdateFieldGPUSmoothingKernels != nullptr)
+ for (unsigned d = 0; d < ImageDimension; ++d)
  {
- for (unsigned d = 0; d < ImageDimension; ++d)
- {
- os << indent.GetNextIndent() << m_UpdateFieldGPUSmoothingKernels[d] << std::endl;
- }
+ os << indent.GetNextIndent() << m_UpdateFieldGPUSmoothingKernels[d] << std::endl;
  }
 
  os << indent << "ImageSizes: ";