{glib,gio}/cxx: Use nullptr to fix missing sentinel on musl
Fixes: ../gio/tests/cxx.cpp: In function 'int main(int, char**)': ../gio/tests/cxx.cpp:62:15: error: missing sentinel in function call [-Werror=format=] 62 | g_test_init (&argc, &argv, NULL); | ~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~
This change was originally PR'd in !2439 (merged) However a different one was PR'd in !3294 (merged)
This caused build failures in Void Linux on musl: https://github.com/void-linux/void-packages/actions/runs/4485990212/jobs/7888064519
Changing NULL to nullptr inside #if G_CXX_STD_CHECK_VERSION (11)
does fix this issue for me.