🎩 What? Why?

Please describe your pull request.
On the meetings registration page, in the admin. There's a crash when registrations are enabled and either available_slots or reserved_slots are blank and the form is submitted.

📌 Related Issues

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• Related to #?
• Fixes #?

Testing

Describe the best way to test or validate your PR.

1. Edit a meeting and set its "Registration type" to "On this platform". Save
2. Go to meetings registrations page in the admin
3. Mark the check "Registrations enabled"
4. Delete the content of "Available slots for this meeting" and "Reserved slots"
5. Save
6. The form is rendered again with error and both fields are marked as "can't be blank"

📋 Checklist

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