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Defensively check javaUtilOptionalEmpty

Issue: SPR-11888
Defensively check javaUtilOptionalEmpty

Issue: SPR-11888
MBeanExportConfiguration's SpecificPlatform properly calls afterPropertiesSet

Also makes the nested SpecificPlatform enum public for Spring Boot to reuse it.

Issue: SPR-11877
WebSphereUowTransactionManager exposes SmartTransactionObject

Issue: SPR-11876