verify if a PipelineState could hold a nil pipeline

[colinsurprenant]

This is a followup of issue #10346 review comment #10346 (comment)

There is currently a check for a nil pipeline condition such as

logstash/logstash-core/lib/logstash/instrument/periodic_poller/dlq.rb

Lines 13 to 18 in 7b6d60c

	pipelines = @agent.running_user_defined_pipelines
	pipelines.each do |_, pipeline|
	unless pipeline.nil?
	pipeline.collect_dlq_stats
	end
	end

This check should not be necessary but a more thorough review should be done to make sure.