lockdown: Only set SystemBUID in pair_record_generate() if it actually contains a value
lockdown: Only set SystemBUID in pair_record_generate() if it actually contains a value

This will prevent a crash in libplist when plist_new_string() will be called with
a NULL pointer.

libimobiledevice-win32/libimobiledevice,Tatsh/libimobiledevice,Tatsh/libimobiledevice,upbit/libimobiledevice,patweb/libimobiledevice,Tatsh/libimobiledevice,libimobiledevice-win32/libimobiledevice,libimobiledevice-win32/libimobiledevice,upbit/libimobiledevice,Tatsh/libimobiledevice,patweb/libimobiledevice,libimobiledevice/libimobiledevice,patweb/libimobiledevice,libimobiledevice/libimobiledevice,libimobiledevice-win32/libimobiledevice,kalev/libimobiledevice,kalev/libimobiledevice,kalev/libimobiledevice,upbit/libimobiledevice