[Sentry][Agent] Add validation on first_day presence

plage_ouverture: {
   end_time: 18:00, 
   first_day: 2019-1201, 
   lieu_id: 41, 
   motif_ids: [237], 
   recurrence: null, 
   start_time: 09:00, 
   title: Palm Beach
}

start_time = Date.parse('2019-1201') # return Invalid Date --> nil
plage_ouverture.valid? == true

https://sentry.io/organizations/lapins/issues/1350411511/?environment=production&project=1811205&referrer=alert_email