[Java] Dummy NPEs in DefaultAllocationManagerOption

Looking at DefaultAllocationManagerOption [1], I see that we could prevent dummy NPEs when trying to convert an often not-set environment option into enums.
In my case, it was a bit painful to have this error, even if properly caught, because it was polluting debug when looking for unexpected NPEs. This was raising false positives.
 
I was wondering if you were open to a pull request to change this little inconvenience?
 
[1]

**Reporter**: [Olivier Peyrusse](https://issues.apache.org/jira/browse/ARROW-13697)

_{**Note**: *This issue was originally created as [ARROW-13697](https://issues.apache.org/jira/browse/ARROW-13697). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

Commencez par lire java/memory/memory-core/src/main/java/org/apache/arrow/memory/DefaultAllocationManagerOption.java autour des lignes 64-82, en vous concentrant sur la conversion des options d’environnement non définies en enums. Confirmez le comportement actuel pour une option non définie et vérifiez que le traitement qui en résulte ne produit plus de NullPointerExceptions factices, tout en préservant le comportement attendu de l’option.

Rédigé par le modèle d'indexation à partir du texte de l'issue.