[Java] Dummy NPEs in DefaultAllocationManagerOption

[asfimport]

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] https://github.com/apache/arrow/blob/master/java/memory/memory-core/src/main/java/org/apache/arrow/memory/DefaultAllocationManagerOption.java#L64-L82

Reporter: Olivier Peyrusse

_{Note: This issue was originally created as ARROW-13697. Please see the migration documentation for further details.}