[8.0] empty string seems on calendar rules !

[malatestay]

Traceback (most recent call last):
File "/opt/odoo/openerp/http.py", line 530, in _handle_exception
return super(JsonRequest, self)._handle_exception(exception)
File "/opt/odoo/openerp/http.py", line 567, in dispatch
result = self._call_function(*_self.params)
File "/opt/odoo/openerp/http.py", line 303, in _call_function
return checked_call(self.db, *args, *_kwargs)
File "/opt/odoo/openerp/service/model.py", line 113, in wrapper
return f(dbname, _args, *_kwargs)
File "/opt/odoo/openerp/http.py", line 300, in checked_call
return self.endpoint(_a, *_kw)
File "/opt/odoo/openerp/http.py", line 796, in call
return self.method(_args, *_kw)
File "/opt/odoo/openerp/http.py", line 396, in response_wrap
response = f(_args, *_kw)
File "/opt/odoo/addons/calendar/controllers/main.py", line 61, in notify
res = registry.get("calendar.alarm_manager").get_next_notif(cr, uid, context=context)
File "/opt/odoo/openerp/api.py", line 241, in wrapper
return old_api(self, _args, *_kwargs)
File "/opt/odoo/addons/calendar/calendar.py", line 496, in get_next_notif
for one_date in self.pool.get("calendar.event").get_recurrent_date_by_event(cr, uid, curEvent, context=context):
File "/opt/odoo/openerp/api.py", line 241, in wrapper
return old_api(self, _args, *_kwargs)
File "/opt/odoo/addons/calendar/calendar.py", line 712, in get_recurrent_date_by_event
rset1 = rrule.rrulestr(str(event.rrule), dtstart=startdate, forceset=True)
File "/usr/lib/python2.7/dist-packages/dateutil/rrule.py", line 1093, in call
return self._parse_rfc(s, **kwargs)
File "/usr/lib/python2.7/dist-packages/dateutil/rrule.py", line 994, in _parse_rfc
raise ValueError, "empty string"
ValueError: empty string

[xmo-odoo]

Dupe of #5313