CDK-802: Fix Nullable annotation with AllowNulls.

This copies the fix for AVRO-1604 into Kite's AllowNulls. The method has
been renamed to avoid confusion with the static method makeNullable in
ReflectData.
Merge pull request #272 from rdblue/CDK-802-backport-AVRO-1604

CDK-802: Fix Nullable annotation with AllowNulls.