Cleanup: Replace NULL with nullptr

At several places, NULL and nullptr were mixed. This commit cleans this up and switches NULL to nullptr where appropriate.

Closes #13168.

PiperOrigin-RevId: 362043567
diff --git a/src/main/cpp/archive_utils.cc b/src/main/cpp/archive_utils.cc
index ab6445f..b20b6cb 100644
--- a/src/main/cpp/archive_utils.cc
+++ b/src/main/cpp/archive_utils.cc
@@ -37,7 +37,7 @@
 
   // Scan the zip file "archive_path" until a file named "stop_entry" is seen,
   // then stop.
-  // If entry_names is not null, it receives a list of all file members
+  // If entry_names is not nullptr, it receives a list of all file members
   // up to and including "stop_entry".
   // If a callback is given, it is run with the name and contents of
   // each such member.