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/clearenv.cpp b/libc/bionic/clearenv.cpp
index c70cc3e..01c9c39 100644
--- a/libc/bionic/clearenv.cpp
+++ b/libc/bionic/clearenv.cpp
@@ -31,9 +31,9 @@
 
 int clearenv() {
   char** e = environ;
-  if (e != NULL) {
+  if (e != nullptr) {
     for (; *e; ++e) {
-      *e = NULL;
+      *e = nullptr;
     }
   }
   return 0;