[gnote] Use nullptr instead

[Aurimas Černius <aurimasc src gnome org>]

commit 77f7f7abb888c01c9758f38d144604a185398927
Author: Aurimas Černius <aurisc4 gmail com>
Date:   Fri Apr 30 21:25:14 2021 +0300

    Use nullptr instead

 src/abstractaddin.cpp | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)
---
diff --git a/src/abstractaddin.cpp b/src/abstractaddin.cpp
index 209dddfb..961b35ba 100644
--- a/src/abstractaddin.cpp
+++ b/src/abstractaddin.cpp
@@ -1,7 +1,7 @@
 /*
  * gnote
  *
- * Copyright (C) 2010,2019 Aurimas Cernius
+ * Copyright (C) 2010,2019,2021 Aurimas Cernius
  * Copyright (C) 2009 Hubert Figuiere
  *
  * This program is free software: you can redistribute it and/or modify
@@ -22,12 +22,11 @@
 
 
 #include "abstractaddin.hpp"
-#include <cstddef>
 
 namespace gnote {
 
   AbstractAddin::AbstractAddin()
-    : m_gnote(NULL)
+    : m_gnote(nullptr)
     , m_disposing(false)
   {
   }