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

Created by: 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.

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