[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.*}

まず java/memory/memory-core/src/main/java/org/apache/arrow/memory/DefaultAllocationManagerOption.java の64-82行付近を読み、設定されていない環境オプションをenumsに変換する処理に注目してください。設定されていないオプションに対する現在の動作を確認し、その結果の処理によってダミーのNullPointerExceptionsが発生しなくなっていることを、期待されるオプションの動作を維持しながら検証してください。

索引モデルが issue の本文から書いたものです。