fix(backend): pages/updateのnameの重複チェックはnameプロパティがある時のみ行うように

[kakkokari-gtyih]

What

かいてあるとおり

Why

Fix #15097

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
差分はありません。
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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 11 lines in your changes missing coverage. Please review.

Project coverage is 39.45%. Comparing base (d2e22f9) to head (e4f805a).
Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
...s/backend/src/server/api/endpoints/pages/update.ts	0.00%	11 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop   #15104      +/-   ##
===========================================
- Coverage    39.45%   39.45%   -0.01%     
===========================================
  Files         1566     1566              
  Lines       198811   198813       +2     
  Branches      3638     3635       -3     
===========================================
- Hits         78447    78436      -11     
- Misses      119793   119806      +13     
  Partials       571      571              

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[mi-gh-maintainer]

Thank you 🙏