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/tdestroy.cpp b/libc/bionic/tdestroy.cpp
index 2968ff9..6606c56 100644
--- a/libc/bionic/tdestroy.cpp
+++ b/libc/bionic/tdestroy.cpp
@@ -27,7 +27,7 @@
 // This is a GNU extension, not available from BSD.
 void tdestroy(void* root, void (*destroy_func)(void*)) {
   node_t* root_node = reinterpret_cast<node_t*>(root);
-  if (root_node == NULL) {
+  if (root_node == nullptr) {
     return;
   }
   if (root_node->llink) {