plugin-common的请求处理系统，返回undefined无法通过类型检查

[DGCK81LNN]

文档中说请求处理函数返回undefined可以忽略请求，但实际开发时这样通不过类型检查。

修改了RequestHandler的定义，允许返回void类型。

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（抱歉刚才没写完说明误开了pr

[codecov]

Codecov Report

Merging #254 (04daa5b) into master (11a5443) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #254   +/-   ##
=======================================
  Coverage   92.37%   92.37%           
=======================================
  Files          57       57           
  Lines       10919    10919           
  Branches     2245     2245           
=======================================
  Hits        10086    10086           
  Misses        833      833           

Impacted Files	Coverage Δ
packages/plugin-common/src/handler.ts	100.00% <100.00%> (ø)

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[shigma]

感觉因为 koishi 官仓没有使用 strictNullChecks 所以不会报错，不过确实可能有这种需求，感谢