fix reaction-button-show-null-als-signer

Closes #1712 (closed)

What does this MR do?

fixes a display bug for reactions of deleted accounts

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

confident

Screenshots (if applicable)

Checklist

• 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 in the release notes to /release-notes/YYYY-MM.md

Closes #1712 (closed)