[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.*}

Start by reading java/memory/memory-core/src/main/java/org/apache/arrow/memory/DefaultAllocationManagerOption.java around lines 64-82, focusing on conversion of unset environment options into enums. Confirm the current behavior for an unset option and verify that the resulting handling no longer produces dummy NullPointerExceptions while preserving the expected option behavior.

Written by the indexing model from the issue text.