[zattoo] It won't work with None in Python 3.6, set always a default date instead of None.
[zattoo] It won't work with None in Python 3.6, set always a default date instead of None.

Fixed streamlink/streamlink#1273

back-to/streamlink,streamlink/streamlink,streamlink/streamlink,bastimeyer/streamlink,wlerin/streamlink,chhe/streamlink,bastimeyer/streamlink,melmorabity/streamlink,gravyboat/streamlink,wlerin/streamlink,back-to/streamlink,beardypig/streamlink,beardypig/streamlink,melmorabity/streamlink,javiercantero/streamlink,gravyboat/streamlink,javiercantero/streamlink,chhe/streamlink