[Java] Fix high severity issues reported by LGTM

[asfimport]

Fixes high severity issues reported by LGTM:

https://lgtm.com/projects/g/apache/arrow/?mode=list&lang=java&severity=error

Reporter: Fokko Driesprong / @Fokko
Assignee: Fokko Driesprong / @Fokko

PRs and other links:

• GitHub Pull Request #6188

_{Note: This issue was originally created as ARROW-7570. Please see the migration documentation for further details.}

[asfimport]

Micah Kornfield / @emkornfield:
Issue resolved by pull request 6188
#6188