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/sched_getcpu.cpp b/libc/bionic/sched_getcpu.cpp
index 1f92e54..f245fba 100644
--- a/libc/bionic/sched_getcpu.cpp
+++ b/libc/bionic/sched_getcpu.cpp
@@ -33,7 +33,7 @@
 
 int sched_getcpu() {
   unsigned cpu;
-  int rc = __getcpu(&cpu, NULL, NULL);
+  int rc = __getcpu(&cpu, nullptr, nullptr);
   if (rc == -1) {
     return -1; // errno is already set.
   }