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/private/bionic_futex.h b/libc/private/bionic_futex.h
index fd68007..b340690 100644
--- a/libc/private/bionic_futex.h
+++ b/libc/private/bionic_futex.h
@@ -51,11 +51,11 @@
 }
 
 static inline int __futex_wake(volatile void* ftx, int count) {
-  return __futex(ftx, FUTEX_WAKE, count, NULL, 0);
+  return __futex(ftx, FUTEX_WAKE, count, nullptr, 0);
 }
 
 static inline int __futex_wake_ex(volatile void* ftx, bool shared, int count) {
-  return __futex(ftx, shared ? FUTEX_WAKE : FUTEX_WAKE_PRIVATE, count, NULL, 0);
+  return __futex(ftx, shared ? FUTEX_WAKE : FUTEX_WAKE_PRIVATE, count, nullptr, 0);
 }
 
 static inline int __futex_wait(volatile void* ftx, int value, const timespec* timeout) {