fix(sync) don't return value from timer callback

[chobits]

Summary

1. timer's handler should not return value
2. If timer's handler return value, which triggers luajit modify this function type, then it will trigger a timerng bug: 2024/12/03 17:07:18 [error] 26989#0: *24 [lua] job.lua:290: execute(): [timer-ng] failed to run timer unix_timestamp=1733216838235.000000; counter=11:meta=@./kong/clustering/rpc/socket.lua:67:Lua(): ...l-bin/build/kong-dev/share/lua/5.1/resty/timerng/job.lua:109: bad argument #1 to 'string_len' (string expected, got nil), context: ngx.timer

Checklist

• The Pull Request has tests
• A changelog file has been created under changelog/unreleased/kong or skip-changelog label added on PR if changelog is unnecessary. README.md
• There is a user-facing docs PR against https://github.com/Kong/docs.konghq.com - PUT DOCS PR HERE

Issue reference

Fix #[issue number]