Refine type hint for cached hardware information

- Updated the type hint for the `_CACHED_HW_INFO` variable to use `Optional[str]` for improved clarity and to indicate that it can be `None`.

This change enhances type safety and readability in the hardware service module.

src/hardware_service.pyx

@@ -1,11 +1,12 @@
 import os
 import platform
 import subprocess
+from typing import Optional
 
 import psutil
 cimport constants
 
-cdef str _CACHED_HW_INFO = None
+cdef Optional[str] _CACHED_HW_INFO = None
 
 
 def _get_cpu():