Cleanup SpotBugs warnings in the codebase

There is a number of SporBugs static analysis warnings in the old parts of the Jenkinsfile Runner codebase. It is possible to see the warnings in the build log. Although there is no severe issues, it would be great to cleanup these warnings and then enforce SpotBugs checks during the build.

List of the current SpotBugs violations: https://ci.jenkins.io/job/jenkinsfile-runner/job/jenkinsfile-runner/job/master/lastSuccessfulBuild/analysis/

Acceptance criteria:

• All SpotBugs violations need to be either fixed or suppressed
• FindBugs failOnError is reenabled in the pom.xml

NOTE: Contributors can submit pull requests for particular issues and issue types, no need to fix everything in a single pull request.