[java] fix default value - empty list []

[wing328]

• fix default value - empty list []
• add tests
• to fix [BUG] [SPRING] incorrect generation of add*Item methods for List-fields in 6.5.0 #15402

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
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  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh
  

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  Commit all changed files.
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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)