[SystemZ][ZOS] Fix __libcpp_thrad_isnull()

[zibi]

Summary

Previous pthread revision https://reviews.llvm.org/D91875 missed a small change for __libcpp_thrad_isnull() required for z/OS.

[zibi]

This has been reviewed before.

libcxx/include/__threading_support

 // Returns non-zero if t1 < t2, otherwise 0
 bool __libcpp_thread_id_less(__libcpp_thread_id t1, __libcpp_thread_id t2)
 {
   return t1 < t2;
 }
 
 // Thread
 bool __libcpp_thread_isnull(const __libcpp_thread_t *__t) {
-  return *__t == __libcpp_thread_t();
+  return __libcpp_thread_get_id(__t) == 0;
 }
 
 int __libcpp_thread_create(__libcpp_thread_t *__t, void *(*__func)(void *),
                            void *__arg)
 {
   return pthread_create(__t, nullptr, __func, __arg);
 }