fix(frontend): correct condition to displayed as system account on moderation page

[zyoshoka]

What

ユーザーのモデレーションページにおいてユーザー名にドットが入っているとシステムアカウントとして表示されてしまう問題を修正します。

Why

#11135 (comment)

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

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 20.25%. Comparing base (41936c1) to head (d9a1b63).
Report is 6 commits behind head on develop.

Files	Patch %	Lines
packages/frontend/src/pages/admin-user.vue	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           develop   #14407       +/-   ##
============================================
- Coverage    39.91%   20.25%   -19.67%     
============================================
  Files         1545      723      -822     
  Lines       190505   100283    -90222     
  Branches      2654     1049     -1605     
============================================
- Hits         76046    20308    -55738     
+ Misses      113898    79434    -34464     
+ Partials       561      541       -20     

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[syuilo]

🙏🏻