Change default for tags argument in gracedb.upload from empty tuple to empty list

gracedb-client's writeLog method expects the tag_name argument to either be a string or a list. The default for gracedb.upload is an empty tuple (see https://git.ligo.org/emfollow/gwcelery/blob/master/gwcelery/tasks/gracedb.py#L76).

This is resulting in gcn circular log messages being uploaded with a "()" tag attached to them:

I will add an additional check into gracedb-client to convert a tuple of tags to a list, but I thought that you may want to change the default from tags=() to tags=[] here as well. Up to you.