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/__gnu_basename.cpp b/libc/bionic/__gnu_basename.cpp
index 1eb3f65..f9f87a6 100644
--- a/libc/bionic/__gnu_basename.cpp
+++ b/libc/bionic/__gnu_basename.cpp
@@ -31,5 +31,5 @@
 
 extern "C" const char* __gnu_basename(const char* path) {
   const char* last_slash = strrchr(path, '/');
-  return (last_slash != NULL) ? last_slash + 1 : path;
+  return (last_slash != nullptr) ? last_slash + 1 : path;
 }