NoMethodError when using in_monitor_agent with a unbuffered plugin

[shuji-koike]

When using in_monitor_agent with out_null plugin a unbuffered plugin (which @buffer is nil), NoMethodError will occur each time http://0.0.0.0:24220/api/plugins is accessed.

Minimal config to reproduce the problem.

<source>
  @type monitor_agent
  bind 0.0.0.0
  port 24220
</source>

<match **>
  @type null
</match>

start fluentd and, kick monitor_agent.

curl http://0.0.0.0:24220/api/plugins

NoMethodError occurs.

2016-12-12 15:16:34 +0900 [warn]: plugin/in_monitor_agent.rb:341:rescue in block in get_monitor_info: unexpected error in monitoring plugins key="buffer_queue_length" plugin=Fluent::Plugin::NullOutput error_class=NoMethodError error="undefined method `queue' for nil:NilClass"
2016-12-12 15:16:34 +0900 [warn]: plugin/in_monitor_agent.rb:341:rescue in block in get_monitor_info: unexpected error in monitoring plugins key="buffer_total_queued_size" plugin=Fluent::Plugin::NullOutput error_class=NoMethodError error="undefined method `stage_size' for nil:NilClass"

[shuji-koike]

Turns out that, not only out_null but some plugins witch @buffer is nil will cause the same problem.
I will update my issue title and comment.

[repeatedly]

Patch is merged.