Fix new errorprone warnings across java-json-tools

[Capstan]

Spurred by #347 .

Since Google's monorepo has warnings-as-errors turned on, I did the same for the java-json-tools repos so that new errors requiring internal patching would be caught upstream before commit. However, the new https://github.com/dependabot suggestion of pulling in 2.4.0 finds new errorprone issues, which causes the builds to fail.

This is a FR to locally pull in 2.4.0, and commit fixes to the issues so the new error prone is clean.

/cc @huggsboson