Fix bugs found by PMD

[merobi-hub]

As per @wslulciuc 's comment in #1976 , bugs found by the static code analysis plugin (PMD) must be fixed before the tool can be added to CI.

To use PMD, run ./gradlew check in the root directory. Reports will be located in api/build/reports/pmd/. The configuration is currently the same as what we used in OpenLineage, but this can be changed.

Related to: #1976