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/bionic/c32rtomb.cpp b/libc/bionic/c32rtomb.cpp
index ebe9cd3..2909d8b 100644
--- a/libc/bionic/c32rtomb.cpp
+++ b/libc/bionic/c32rtomb.cpp
@@ -34,9 +34,9 @@
 
 size_t c32rtomb(char* s, char32_t c32, mbstate_t* ps) {
   static mbstate_t __private_state;
-  mbstate_t* state = (ps == NULL) ? &__private_state : ps;
+  mbstate_t* state = (ps == nullptr) ? &__private_state : ps;
 
-  if (s == NULL) {
+  if (s == nullptr) {
     // Equivalent to c32rtomb(buf, U'\0', ps).
     return mbstate_reset_and_return(1, state);
   }