writeToParcel throws NPE for optional enum fields

The recent change to the Java compiler added generated Android compatible, parcelable Thrift objects.

https://github.com/apache/thrift/commit/f9b8f5e64137248ea2a1b95312916491d23ab94a#diff-80220be8df49fb98fe5f899544d8368dR1569

However, for optional enum fields that are null, getValue() will throw an NPE because no checks are made that the enum is set before being written.