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/mbrtoc16.cpp b/libc/bionic/mbrtoc16.cpp
index 2180516..acea426 100644
--- a/libc/bionic/mbrtoc16.cpp
+++ b/libc/bionic/mbrtoc16.cpp
@@ -60,10 +60,10 @@
 
 size_t mbrtoc16(char16_t* pc16, const char* s, size_t n, mbstate_t* ps) {
   static mbstate_t __private_state;
-  mbstate_t* state = (ps == NULL) ? &__private_state : ps;
+  mbstate_t* state = (ps == nullptr) ? &__private_state : ps;
 
   char16_t __private_pc16;
-  if (pc16 == NULL) {
+  if (pc16 == nullptr) {
     pc16 = &__private_pc16;
   }