fix(answer): fix panic when failed to gen relevant questions

[zwpaper]

[codecov]

Codecov Report

Attention: Patch coverage is 98.93617% with 1 line in your changes missing coverage. Please review.

Project coverage is 63.32%. Comparing base (a208462) to head (9e0a76f).
Report is 16 commits behind head on main.

Files with missing lines	Patch %	Lines
ee/tabby-webserver/src/service/answer.rs	98.78%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3462      +/-   ##
==========================================
+ Coverage   63.19%   63.32%   +0.13%     
==========================================
  Files         193      193              
  Lines       21949    22019      +70     
==========================================
+ Hits        13871    13944      +73     
+ Misses       8078     8075       -3     

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