[JAVA] Fix optional collection initialized with object [fix: #18735]

[mosesonline]

Since 7.5.0 optional field in collections where initialized with an Object (i.e. new ArrayList<>()) but before they where initialized with null. This PR fixes this by using the required-Property to skip returning null and returning a default-Value. (#18735)

I've also added a test for it.

technical committee member: @martin-mfg

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[mosesonline]

The current behavior is compliant with Open API specification.