bpo-36814: ensure os.posix_spawn() handles None (GH-13144)

Fix an issue where os.posix_spawn() would incorrectly raise a TypeError
when file_actions is None.
diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
index 221f710..aa77094 100644
--- a/Modules/posixmodule.c
+++ b/Modules/posixmodule.c
@@ -5465,7 +5465,7 @@
         goto exit;
     }
 
-    if (file_actions != NULL) {
+    if (file_actions != NULL && file_actions != Py_None) {
         /* There is a bug in old versions of glibc that makes some of the
          * helper functions for manipulating file actions not copy the provided
          * buffers. The problem is that posix_spawn_file_actions_addopen does not