[Java] add nullable annotation to pojo template for non-required fields

Created by: kassim

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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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@bbdouglas @sreeshas @jfiala @lukoyanov @cbornet @jeff9finger @karismann @Zomzog

Description of the PR

fixes https://github.com/OpenAPITools/openapi-generator/issues/3264

adds @javax.annotation.Nullable to non-required fields, to enhance IDE feedback and Kotlin-interop

this requires the com.google.code.findbugs:jsr305:3.0.2 dependency for the annotation - I'm not sure where I should adjust the sample dependencies (I adjusted the pom.xml / build.gradle in the sample but the integration test still fails) documentation to inform people this

existing tests are working fine, I don't think additional ones are necessary