[SystemZ][ZOS] Fix __libcpp_thrad_isnull()

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

Reviewed By: zibi

Differential Revision: https://reviews.llvm.org/D96302

GitOrigin-RevId: 48fcce1aea9e783c3fe64482f06a965707e75990
1 file changed