Modernize codebase by replacing NULL with nullptr

Fixes -Wzero-as-null-pointer-constant warning for binder.

Test: m
Bug: 68236239
Change-Id: I8184bd6aa4ebff1bd8c88dad16886e98df853b03
diff --git a/libutils/include/utils/AndroidThreads.h b/libutils/include/utils/AndroidThreads.h
index dab888d..a8d7851 100644
--- a/libutils/include/utils/AndroidThreads.h
+++ b/libutils/include/utils/AndroidThreads.h
@@ -106,7 +106,7 @@
                             const char* threadName = "android:unnamed_thread",
                             int32_t threadPriority = PRIORITY_DEFAULT,
                             size_t threadStackSize = 0,
-                            thread_id_t *threadId = 0)
+                            thread_id_t *threadId = nullptr)
 {
     return androidCreateThreadEtc(entryFunction, userData, threadName,
         threadPriority, threadStackSize, threadId) ? true : false;