Backend/i18n: Consider empty translations as missing

[tristanlabelle]

Weblate can write empty strings to translation files and expects the code to treat them as missing translations rather than as valid empty string translations. This happens with missing plural forms, for example:

  "attendees_count_one": "{{count}} participant",
  "attendees_count_many": "",
  "attendees_count_other": "{{count}} participants",

Testing

Added a test

Backend checklist

• Added tests for any new code or added a regression test if fixing a bug
• Run the backend locally and it works
• Added migrations if there are any database changes, rebased onto develop if necessary for linear migration history

For maintainers

• Maintainers can push commits to my branch
• Maintainers can merge this PR for me

[vercel]

The latest updates on your projects. Learn more about Vercel for GitHub.

Project	Deployment	Actions	Updated (UTC)
couchers	Ready	Preview	Feb 22, 2026 0:01am

[tristanlabelle]

Note to self: will conflict with #7884

[aapeliv]

lgtm