STYLE: ImageSourceCommonGlobals default member initializer, Rule of Zero

Added an in-class default member initializer to the data member of
`ImageSourceCommonGlobals`. Removed its default-constructor, following the C++
"Rule of Zero".

Modules/Core/Common/src/itkImageSourceCommon.cxx

@@ -25,9 +25,7 @@ namespace itk
 
 struct ImageSourceCommonGlobals
 {
-  ImageSourceCommonGlobals() { m_GlobalDefaultSplitter = ImageRegionSplitterSlowDimension::New().GetPointer(); };
-
-  ImageRegionSplitterBase::Pointer m_GlobalDefaultSplitter;
+  ImageRegionSplitterBase::Pointer m_GlobalDefaultSplitter{ ImageRegionSplitterSlowDimension::New().GetPointer() };
 };
 
 itkGetGlobalSimpleMacro(ImageSourceCommon, ImageSourceCommonGlobals, PimplGlobals);