fix(backend): お知らせ作成時に画像URL入力欄を空欄に変更できないのを修正

[eternal-flame-AD]

What

Fixes #14976

Why

Additional info (optional)

Checklist

• [X] Read the contribution guide
• [X] Test working in a local environment
• [ ] (If needed) Add story of storybook
• [X] (If needed) Update CHANGELOG.md
• [X] (If possible) Add tests

[github-actions[bot]]

このPRによるapi.jsonの差分

差分はこちら

```diff --- base +++ head @@ -3219,7 +3219,7 @@ "string", "null" ], - "minLength": 1 + "minLength": 0 }, "icon": { "type": "string", ``` [Get diff files from Workflow Page](https://github.com/misskey-dev/misskey/actions/runs/11884151653)

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 39.97%. Comparing base (eed45c7) to head (fdd43e6). Report is 3 commits behind head on develop.

Files with missing lines	Patch %	Lines
...server/api/endpoints/admin/announcements/create.ts	33.33%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #14990 +/- ## =========================================== - Coverage 39.97% 39.97% -0.01% =========================================== Files 1561 1561 Lines 197256 197257 +1 Branches 3607 3606 -1 =========================================== - Hits 78849 78847 -2 + Misses 117835 117804 -31 - Partials 572 606 +34 ```

:umbrella: View full report in Codecov by Sentry.
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[eternal-flame-AD]

https://github.com/misskey-dev/misskey/actions/runs/11884135456/job/33111877091?pr=14990#step:11:945

I have this problem on my Forgejo CI too (happened before this commit so not related).

[kakkokari-gtyih]

https://github.com/misskey-dev/misskey/actions/runs/11884135456/job/33111877091?pr=14990#step:11:945

I have this problem on my Forgejo CI too (happened before this commit so not related).

I re-ran the CI and now it passed

[syuilo]

🙏