command: fix NoMethodError of --daemon under Windows

[Watson1978]

Which issue(s) this PR fixes:
Fixes #4354

What this PR does / why we need it:
To daemonize the process, the following code will be invoked on Windows:

fluentd/lib/fluent/daemon.rb

Line 13 in 89f0a76

ServerEngine::Daemon.run_server(server_module, worker_module) { Fluent::Supervisor.serverengine_config(params) }

$log variable does not exist because it has not executed Superviser#setup_global_logger method until here.
It will perform a new startup process and prepare a logger in

fluentd/lib/fluent/daemon.rb

Line 13 in 89f0a76

ServerEngine::Daemon.run_server(server_module, worker_module) { Fluent::Supervisor.serverengine_config(params) }

This patch will only avoid the error since a logger is not required this process.

Docs Changes:
Not needed.

Release Note:
The same as the title.

[daipom]

Thanks!

Note:

• When log and logger are nil, the default ServerEngine::Daemon would be set.
  • https://github.com/treasure-data/serverengine/blob/v2.4.0/lib/serverengine/config_loader.rb#L62-L79
• However, it would not be used as the process exits soon after spawning.

[daipom]

LGTM.