Describe the enhancement requested

Part of apache/arrow-java#167

Change our Java client code to allow nullable references to Schema. This is to accomodate systems which may not know this information upfront (e.g. there is a queue, or it is an expensive operation, or it happens later in query processing). We can make references to this field optional, and deprecate non-optional existing usages.

CC: @lidavidm, @kou

Component(s)

Java