Handle case where cache file does not exist.

src/calibre/devices/smart_device_app/driver.py

@@ -730,6 +730,7 @@ class SMART_DEVICE_APP(DeviceConfig, DevicePlugin):
                                 '_metadata_cache.json')
         self.known_uuids = defaultdict(dict)
         self.known_metadata = {}
+        if os.path.exists(cache_file_name):
             with open(cache_file_name, mode='rb') as fd:
                 try:
                     while True: