Solve FindBugs Warnings

[SlowMo24]

see #90

• [ ] etl
• [ ] oshdb
• [ ] oshdb-api
• [ ] oshdb-util
• [ ] oshpbf-parser

[tyrasd]

findbugs is unmaintained (successor is spotbugs, see https://github.com/findbugsproject/findbugs#findbugs-is-now-spotbugs). closing in favour of #92