Add support for passing nullptr to wx pseudo-vararg functions

Allow passing literal nullptr as an argument corresponding to "%p" in
the format string.

See https://github.com/wxWidgets/wxWidgets/pull/1251

Closes #18355.

tests/strings/vararg.cpp

@@ -249,6 +249,13 @@ void VarArgTestCase::ArgsValidation()
     wxString::Format("a string(%s,%s), ptr %p, int %i",
                      wxString(), "foo", "char* as pointer", 1);
 
+#if __cplusplus >= 201103 || wxCHECK_VISUALC_VERSION(10)
+    // Unfortunately we can't check the result as different standard libraries
+    // implementations format it in different ways, so just check that it
+    // compiles.
+    wxString::Format("null pointer is %p", nullptr);
+#endif
+
     // Microsoft has helpfully disabled support for "%n" in their CRT by
     // default starting from VC8 and somehow even calling
     // _set_printf_count_output() doesn't help here, so don't use "%n" at all