Resolve "Invalid cooperation start date in store information dialog"

Closes #1667 (closed)

What does this MR do?

Fix missing check for value "0000-00-00" of store.

How confident are you it won't break things if deployed?

How to test

A. Check unit tests of StoreGateway

B. Manual test

1. Modify store "begin" date to "0000-00-00" (do not set it to null)
2. Checkout branch locally
3. Login as store responsible
4. Open store dialog for modified store
5. Try to modify store (save it)
6. Old behavior would provide an error
7. New behavior will update without problems

Screenshots (if applicable)

Checklist

• added a test, or explain why one is not needed/possible...
• no unrelated changes
• asked someone for a code review
• set a "for:" label to indicate who will be affected by this change
• added to the next milestone (see https://gitlab.com/foodsharing-dev/foodsharing/-/milestones, unless it has a "for:Dev" label)
• added an entry to CHANGELOG.md
• added a short text that can be used in the release notes
• Once your MR has been merged, you are responsible to create a testing issue in the Beta Testing forum: https://foodsharing.de/?page=bezirk&bid=734&sub=forum. Please change the MRs label to "state:Beta testing".
  • Consider writing a detailed description in German.
  • Describe in a few sentences, what should be tested from a user perspective.
  • Also mention different settings (e.g. different browsers, roles, ...) how this change can be tested.
  • Be aware, that also non technical people should understand.

Release notes text in German

Closes #1667 (closed)