feat(backend): Webhook設定でsecretを空に出来るように

[Sayamame-beans]

What

Webhook設定のsecretを空文字列に出来るようにしました。
また、パラメーターとして必須でないように(デフォルトが空文字となるように)しました。

Why

Resolve #11774

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

Patch coverage: 100.00% and no project coverage change.

Comparison is base (e7d30c8) 78.64% compared to head (a7afbd2) 78.64%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop   #11784   +/-   ##
========================================
  Coverage    78.64%   78.64%           
========================================
  Files          923      923           
  Lines        97509    97509           
  Branches      7739     7740    +1     
========================================
+ Hits         76682    76685    +3     
+ Misses       20827    20824    -3     

Files Changed	Coverage Δ
...kend/src/server/api/endpoints/i/webhooks/create.ts	73.75% <100.00%> (ø)
...kend/src/server/api/endpoints/i/webhooks/update.ts	71.60% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[syuilo]

🙏