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

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

[github-actions]

このPRによるapi.jsonの差分

差分はこちら

--- base
+++ head
@@ -3219,7 +3219,7 @@
                       "string",
                       "null"
                     ],
-                    "minLength": 1
+                    "minLength": 0
                   },
                   "icon": {
                     "type": "string",

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[codecov]

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 1 commits behind head on develop.

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

Additional details and impacted files

@@             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     

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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).