[Java] add useNullForUnknownEnumValue option

Created by: jmini

PR checklist

• Read the contribution guidelines.
• 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 and ./bin/security/{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\.
• Filed the PR against the correct branch: master, 3.2.x, 4.0.x. Default: master.
• Java Technical Committee: @bbdouglas (2017/07) @JFCote (2017/08) @sreeshas (2017/08) @jfiala (2017/08) @lukoyanov (2017/09) @cbornet (2017/09) @jeff9finger (2018/01)

cc: @TehBakker, @slarti-b, @wing328, @cbornet

Description of the PR

Fixes #625 (closed)

This PR introduce a new option useNullForUnknownEnumValue.

• When false the generated code throws an exception when the value is unknown (not present in the enum)
• When true (3.1.x and 3.0.x behavior): unknown values are mapped to null

Migration guide is updated with this PR.

2 Java samples are still using the old behavior:

• bin/java-petstore-jersey1.sh
• bin/java-petstore-jersey2.sh