Fixed an issue on some compilers with implicit conversion from nullptr to Ptr

- also cleaned up some warnings to do with CATCH_NULL

projects/SelfTest/SurrogateCpps/catch_xmlwriter.cpp

@@ -1,2 +1,4 @@
 // This file is only here to verify (to the extent possible) the self sufficiency of the header
+#include "catch_suppress_warnings.h"
 #include "catch_xmlwriter.hpp"
+#include "catch_reenable_warnings.h"