fix: service system prompt init as an empty string

[woorui]

Description

fix bug below:

[vercel]

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1 Ignored Deployment

Name	Status	Preview	Updated (UTC)
yomo	⬜️ Ignored (Inspect)		Apr 29, 2024 10:57am

[codecov]

Codecov Report

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

Project coverage is 64.80%. Comparing base (cd899d9) to head (6171890).

Files	Patch %	Lines
pkg/bridge/ai/service.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #804      +/-   ##
==========================================
- Coverage   64.83%   64.80%   -0.03%     
==========================================
  Files          46       46              
  Lines        2650     2651       +1     
==========================================
  Hits         1718     1718              
- Misses        818      819       +1     
  Partials      114      114              

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