Fix deprecation for null in strtolower

Closes #1569 (closed)

What does this MR do?

Add a check that not null is passed to strtolower but emptystring instead

I found no further usage, where null could be expected

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

Sure

How to test

Tests for Mails are missing

links

https://yunity.slack.com/archives/C03N7TP7ASY/p1688830391718469

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