COMP: Work around GCC <= 9.1 "error: invalid application of ‘sizeof’..."

Worked around a GCC bug which produced the following error:

> unique_ptr.h:79:16: error: invalid application of ‘sizeof’ to incomplete type

The issue #3643
"Compilation fails with GCC 8.4." was reported by Max Aehle. The GCC bug appears
to be fixed with GCC 9.2.

The workaround (removing "`{ nullptr }`") was also suggested by Max Aehle.

Modules/Core/Common/include/itkObject.h

@@ -274,7 +274,7 @@ class ITKCommon_EXPORT Object : public LightObject
 
   /** Implementation class for Subject/Observer Pattern.
    * This is only allocated if used. */
-  std::unique_ptr<SubjectImplementation> m_SubjectImplementation{ nullptr };
+  std::unique_ptr<SubjectImplementation> m_SubjectImplementation;
 
   /**
    * Implementation for holding Object MetaData