STYLE: Replace ITK_NULLPTR with nullptr

This commit replaces occurences of the ITK_NULLPTR macro with nullptr. This one-liner was used to perform the update:

git grep -l "ITK_NULLPTR" | fgrep -v CMakeLists.txt |fgrep -v .cmake | xargs sed -i '' -e "s/ITK_NULLPTR/nullptr/g"

It was adapted from https://github.com/InsightSoftwareConsortium/ITK/blob/master/Utilities/ITKv5Preparation/ReplaceITK_NULLPTRMacroNames.sh

Updated information can be found in the wiki: https://www.slicer.org/wiki/Documentation/Nightly/Developers/Tutorials/MigrationGuide/ObsoleteCodeRemoval#C.2B.2B11:_Update_source_code_to_use_nullptr