[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.*}

Inizia leggendo java/memory/memory-core/src/main/java/org/apache/arrow/memory/DefaultAllocationManagerOption.java intorno alle righe 64-82, concentrandoti sulla conversione delle opzioni dell’ambiente non impostate in enums. Conferma il comportamento attuale per un’opzione non impostata e verifica che la gestione risultante non produca più NullPointerExceptions fittizie, preservando al contempo il comportamento previsto dell’opzione.

Scritto dal modello di indicizzazione a partire dal testo della issue.