[Java] Add @javax.annotation.Nonnull to required getters

[marcelstoer]

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For the Kotlin->Java interoperability (i.e. generated Java code used in Kotlin project) it is paramount that required properties be tagged with JSR-305 @Nonnull annotations. The Kotlin compiler honors those annotations: https://kotlinlang.org/docs/java-interop.html#jsr-305-support. Hence, they need to be generated in addition to the Bean Validation @NotNull.

@bbdouglas @sreeshas @jfiala @lukoyanov @cbornet @jeff9finger @karismann @Zomzog @lwlee2608 @nmuesch

[GithubUser8080]

Why did you add this even if bean validation is not selected?