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/CallStack.h b/libutils/include/utils/CallStack.h
index 9622142..0c1b875 100644
--- a/libutils/include/utils/CallStack.h
+++ b/libutils/include/utils/CallStack.h
@@ -49,13 +49,13 @@
     // Dump a stack trace to the log using the supplied logtag.
     void log(const char* logtag,
              android_LogPriority priority = ANDROID_LOG_DEBUG,
-             const char* prefix = 0) const;
+             const char* prefix = nullptr) const;
 
     // Dump a stack trace to the specified file descriptor.
-    void dump(int fd, int indent = 0, const char* prefix = 0) const;
+    void dump(int fd, int indent = 0, const char* prefix = nullptr) const;
 
     // Return a string (possibly very long) containing the complete stack trace.
-    String8 toString(const char* prefix = 0) const;
+    String8 toString(const char* prefix = nullptr) const;
 
     // Dump a serialized representation of the stack trace to the specified printer.
     void print(Printer& printer) const;