Fix FindBugs violations

Currently FindBugs is simply disabled on the whole plugin. There are many violations:

[INFO] --- findbugs-maven-plugin:3.0.5:check (findbugs) @ ec2 ---
[INFO] BugInstance size is 87
[INFO] Error size is 0
[INFO] Total bugs: 87

Those should likely be fixed. Even more if we use in the core list of plugins for Jenkins Evergreen.

cc rtyler FYI