Finite pipelines which have already stopped throws error when deleted

*Original comment by @suyograo:*

To reproduce

logstash.yml
```
xpack.management.enabled: true
xpack.management.elasticsearch.url: "http://localhost:9200"
xpack.management.elasticsearch.username: elastic
xpack.management.elasticsearch.password: changeme
xpack.management.pipeline.id: ["generator", "apache"]
```

Generator config in UI:

```
input {
generator { count => 1 }
}
output {
file { path => '/tmp/foo" }
}
```

This pipeline generates one event and shuts down. When you now delete this pipeline from the UI, you get these errors:

```sh
[2017-08-03T11:57:06,762][INFO ][logstash.pipeline ] Pipeline terminated {"pipeline.id"=>"generator"}
[2017-08-03T11:57:06,765][INFO ][logstash.agent ] Pipelines running {:count=>1, :pipelines=>[".monitoring-logstash"]}
[2017-08-03T11:57:06,801][ERROR][logstash.agent ] An exception happened when converging configuration {:exception=>NoMethodError, :message=>"undefined method `system?' for nil:NilClass", :backtrace=>["/Users/suyog/bin/logstash-6.0.0-beta1/vendor/bundle/jruby/2.3.0/gems/x-pack-6.0.0.beta1-java/lib/monitoring/inputs/metrics.rb:158:in `update_pipeline_state'", "/Users/suyog/bin/logstash-6.0.0-beta1/vendor/bundle/jruby/2.3.0/gems/x-pack-6.0.0.beta1-java/lib/monitoring/inputs/metrics.rb:71:in `block in pipeline_started'", "/Users/suyog/bin/logstash-6.0.0-beta1/vendor/bundle/jruby/2.3.0/gems/x-pack-6.0.0.beta1-java/lib/license_checker/licensed.rb:76:in `with_license_check'", "/Users/suyog/bin/logstash-6.0.0-beta1/vendor/bundle/jruby/2.3.0/gems/x-pack-6.0.0.beta1-java/lib/monitoring/inputs/metrics.rb:70:in `pipeline_started'", "/Users/suyog/bin/logstash-6.0.0-beta1/logstash-core/lib/logstash/event_dispatcher.rb:34:in `block in fire'", "/Users/suyog/bin/logstash-6.0.0-beta1/logstash-core/lib/logstash/event_dispatcher.rb:32:in `fire'", "/Users/suyog/bin/logstash-6.0.0-beta1/logstash-core/lib/logstash/agent.rb:390:in `block in dispatch_events'", "org/jruby/RubyHash.java:1343:in `each'", "/Users/suyog/bin/logstash-6.0.0-beta1/logstash-core/lib/logstash/agent.rb:383:in `dispatch_events'", "/Users/suyog/bin/logstash-6.0.0-beta1/logstash-core/lib/logstash/agent.rb:171:in `converge_state_and_update'", "/Users/suyog/bin/logstash-6.0.0-beta1/logstash-core/lib/logstash/agent.rb:105:in `block in execute'", "/Users/suyog/bin/logstash-6.0.0-beta1/vendor/bundle/jruby/2.3.0/gems/stud-0.0.23/lib/stud/interval.rb:18:in `interval'", "/Users/suyog/bin/logstash-6.0.0-beta1/logstash-core/lib/logstash/agent.rb:94:in `execute'", "/Users/suyog/bin/logstash-6.0.0-beta1/logstash-core/lib/logstash/runner.rb:346:in `block in execute'", "/Users/suyog/bin/logstash-6.0.0-beta1/vendor/bundle/jruby/2.3.0/gems/stud-0.0.23/lib/stud/task.rb:24:in `block in initialize'"]}
[2017-08-03T11:57:06,827][ERROR][logstash.inputs.metrics ] Failed to create monitoring event {:message=>"undefined method `system?' for nil:NilClass", :error=>"NoMethodError"}
```