Empty time values should be stored as NULL

[Ryan J Ollos]

Empty time values in the ticket system are stored as 0. They should probably be stored as NULL instead. Example: Version.update​

sqlite> SELECT time FROM version;
0
1496041616000000
1472327798000000
0
0

The issue is caused by behavior of to_utimestamp​. We should add a helper function and use in trac.ticket.model:

def _to_timestamp(time):
    return to_utimestamp(time) if time else None