Modernize codebase by replacing NULL with nullptr

Fixes -Wzero-as-null-pointer-constant warning.

Test: m
Bug: 68236239
Change-Id: I5b4123bc6709641315120a191e36cc57541349b2
diff --git a/libc/private/ScopedReaddir.h b/libc/private/ScopedReaddir.h
index 3d77a40..1a59e1a 100644
--- a/libc/private/ScopedReaddir.h
+++ b/libc/private/ScopedReaddir.h
@@ -31,13 +31,13 @@
   }
 
   ~ScopedReaddir() {
-    if (dir_ != NULL) {
+    if (dir_ != nullptr) {
       closedir(dir_);
     }
   }
 
   bool IsBad() {
-    return dir_ == NULL;
+    return dir_ == nullptr;
   }
 
   dirent* ReadEntry() {