[Java][jersey2] Add null type support to oneOf/anyOf

[wing328]

• Add null type support to oneOf/anyOf
• tested locally with a spec using oneOf and "type: null"

To fix #6257

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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) @bkabrda (2020/01)

[wing328]

Tested with https://raw.githubusercontent.com/CiscoM31/intersight-openapi/master/intersight-openapi-v3.yaml and the code compiles without issues.