fixed ValueError: max() arg is an empty sequence

I get an error when trying to sync:

Traceback (most recent call last):
  File "/home/buster/lycheesync/main.py", line 85, in <module>
    main(conf_data)
  File "/home/buster/lycheesync/main.py", line 16, in main
    s.sync()
  File "/home/buster/lycheesync/lycheesyncer.py", line 317, in sync
    self.updateAlbumsDate(albums)
  File "/home/buster/lycheesync/lycheesyncer.py", line 216, in updateAlbumsDate
    maxdate = max(photo.sysdate for photo in a['photos'])
ValueError: max() arg is an empty sequence


That one seems to fix it (although i don't know if the function should set some date or not.. when it doesn't it seems to work fine though).