tests/cxx: Do not assume that NULL is a pointer type starting with C++11

Many toolchain did not change the definition of NULL to avoid introducing
breaking changes in existing codebases. For example, on Windows NULL is
0 (int) regardless of the C++ standard in use.

Fixes the following warnings on CLang when compiling for Windows:

  ../glib/glib/tests/cxx.cpp:539:34: warning: missing sentinel in function call [-Wsentinel]
    g_test_init (&argc, &argv, NULL);
                                   ^
                                   , nullptr
  ../glib/glib/gtestutils.h:298:9: note: function has been explicitly marked sentinel here
  void    g_test_init                     (int            *argc,
          ^

  ../glib/gio/tests/cxx.cpp:62:34: warning: missing sentinel in function call [-Wsentinel]
    g_test_init (&argc, &argv, NULL);
                                   ^
                                   , nullptr
  ../glib/glib/gtestutils.h:298:9: note: function has been explicitly marked sentinel here
  void    g_test_init                     (int            *argc,
          ^

gio/tests/cxx.cpp

@@ -59,7 +59,7 @@ int
 main (int argc, char **argv)
 {
 #if G_CXX_STD_CHECK_VERSION (11)
-  g_test_init (&argc, &argv, NULL);
+  g_test_init (&argc, &argv, nullptr);
 #else
   g_test_init (&argc, &argv, static_cast<void *>(NULL));
 #endif

glib/tests/cxx.cpp

@@ -536,7 +536,7 @@ int
 main (int argc, char *argv[])
 {
 #if G_CXX_STD_CHECK_VERSION (11)
-  g_test_init (&argc, &argv, NULL);
+  g_test_init (&argc, &argv, nullptr);
 #else
   g_test_init (&argc, &argv, static_cast<void *>(NULL));
 #endif