Invalid cooperation start date in store information dialog
Summary
For our Test-Store https://foodsharing.de/?page=fsbetrieb&id=13948, the start date of the cooperation is set to "0000-00-00" on production. The API at https://beta.foodsharing.de/api/stores/13948/information returns it as cooperationStart: -0001-11-30
, which results in the new store information modal not showing any date and complaining on submit. This might be a problem with other stores too. In the database the value can be null. In this case, there should at least be a better error message.