Update LogstashWriter.java to fix "Jenkins.instance is missing" error

[tranceporter]

Image above explains the solution for the issue https://issues.jenkins.io/browse/JENKINS-71693?jql=project%20%3D%20JENKINS%20AND%20component%20%3D%20logstash-plugin

Testing done

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue

[jakub-bochenski]

I think we should include the changes in !146

This change looks useful: https://github.com/jenkinsci/logstash-plugin/pull/138/files#diff-13917fce5a3d9038026493f94c39a0ad41afc4084922449b8d4fb06ffc107959

And we sure can use the additional tests that are added

[Seros]

Hi, how is it going? Can we support with anything to have this issue fixed soon?

[jakub-bochenski]

Combining this PR with !138 should do. You are free to contribute :)