[java-client] set collection to null when not required

[jmini]

PR checklist

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• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\. If contributing template-only or documentation-only changes which will change sample output, be sure to build the project first.
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Description of the PR

Fixes #3585

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[jmini]

cc Java technical committee:
@bbdouglas (2017/07) @sreeshas (2017/08) @jfiala (2017/08) @lukoyanov (2017/09) @cbornet (2017/09) @jeff9finger (2018/01) @karismann (2019/03) @Zomzog (2019/04)

[wing328]

@jmini I remember working on something similar and use nullable to determine the default value (null or empty list)

[wing328]

Related discussion/issue: #1861

[jmini]

I remember working on something similar and use nullable to determine the default value (null or empty list)

As I wrote in issue #3585 this PR is aligning Java/pojo.mustache with JavaJaxRS/pojo.mustache

I will contribute to the discussion in #1861

[wing328]

As I wrote in issue #3585 this PR is aligning Java/pojo.mustache with JavaJaxRS/pojo.mustache

Yes, I read that before leaving my comment. I'm not against this PR. Just want to share I've worked on something similar before with the nullable attribute.