No warning email is sent for pickups that have 0 slots

Summary

Since short time, the mails "Warnung: Für die nächste Abholung..." notifying you about empty slots the night before the fetch are not send to BIEBs anymore. There are ~~336 - 557 mails being sent every night.

You can execute this with:
./scripts/docker-compose run --rm --no-deps app php -f run.php Maintenance warnings

And check mails with: http://localhost:18084/#/

Explanation

Due to the feature to "overwrite" pickup slots with 0 slots the e-mail reminder for pickups needs to be corrected. Pickup slots with 0 slots should not trigger an e-mail.

Steps to reproduce

0. Be a BIEB for a store.
1. create a slot for next day.
2. leave it empty
3. wait till midnight (or a bit longer)

What is the current bug behavior?

You should receive a mail with a warning.

What is the expected correct behavior?

No mail received.

Proposal

if pickupslots = 0 don't send mail.

Relevant error messages and/or screenshots