Style: Replaces uses of 0/NULL by nullptr (C++11)

Using clang-tidy's `modernize-use-nullptr`.
https://clang.llvm.org/extra/clang-tidy/checks/modernize-use-nullptr.html

editor/dictionary_property_edit.cpp

@@ -90,7 +90,7 @@ void DictionaryPropertyEdit::edit(Object *p_obj, const StringName &p_prop) {
 Node *DictionaryPropertyEdit::get_node() {
 	Object *o = ObjectDB::get_instance(obj);
 	if (!o)
-		return NULL;
+		return nullptr;
 
 	return cast_to<Node>(o);
 }