CI: Flaky test failure in ForwardOutputTest with `TypeError: wrong argument type nil (expected Data)`

[Watson1978]

Describe the bug

We encountered a flaky test failure when running ForwardOutputTest.
The error occurs very rarely, but it seems to happen when the following two tests are executed in sequence:

1. test: Create new connection per send_data
2. test: Node with security is thread-safe on multi threads

The error indicates a TypeError inside cool.io's event loop processing.

Logs

1) Error: test: Node with security is thread-safe on multi threads(ForwardOutputTest): TypeError: wrong argument type nil (expected Data)
C:/hostedtoolcache/windows/Ruby/3.2.9/x64/lib/ruby/gems/3.2.0/gems/cool.io-1.9.0/lib/cool.io/loop.rb:88:in `run_once'
C:/hostedtoolcache/windows/Ruby/3.2.9/x64/lib/ruby/gems/3.2.0/gems/cool.io-1.9.0/lib/cool.io/loop.rb:88:in `run'
D:/a/fluentd/fluentd/lib/fluent/plugin_helper/event_loop.rb:93:in `block in start'
D:/a/fluentd/fluentd/lib/fluent/plugin_helper/thread.rb:78:in `block in thread_create'

To Reproduce

Run CI

Expected behavior

No error

Your Environment

- Fluentd version:
- Package version:
- Operating system:
- Kernel version:

Your Configuration

N/A

Your Error Log

See above

Additional context

No response

[Watson1978]

Reported to socketry/cool.io#87