Fix for missing serial_number on macOS

[pavel-kokolemin]

CFStringGetCStringPtr was used incorrectly on macOS in get_string_property.

From https://developer.apple.com/documentation/corefoundation/1542133-cfstringgetcstringptr :

This function either returns the requested pointer immediately, with no memory allocations and no copying, in constant time, or returns NULL. If the latter is the result, call an alternative function such as the [CFStringGetCString(::::)] function to extract the characters.

As the result serial_number is some cases was empty. After switching to CFStringGetCString it now always set correctly .

[jessebraham]

Good catch, thank you!