[BUG][java] required vs optional JSON array validation (#13774)

[ashatch]

@wing328 a JSON array related fix. I'd be interested in your thoughts on whether this style is okay or whether I should aim to collapse the conditionals more? Thanks!

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[wing328]

@ashatch I think it's fine. Thanks for the PR. I'll try to run some tests later.

cc @bbdouglas (2017/07) @sreeshas (2017/08) @jfiala (2017/08) @lukoyanov (2017/09) @cbornet (2017/09) @jeff9finger (2018/01) @karismann (2019/03) @Zomzog (2019/04) @lwlee2608 (2019/10)