Fix ion null deserialization

[MartinGian]

Fixes the IonValueDeserializer to be able to deserialize serialized null values correctly. Ion does have first class null values so a deserialized null value should never end up being a Java null value.

There is more information about this change in #295

[mcliedtke]

Changes LGTM. @cowtowncoder, do you mind merging? I think 2.13 would likely make the most sense.

[cowtowncoder]

Will merge (I have some concerns as to whether this will work reliably due to reliance on underlying JsonParser -- but there are tests and so perhaps it does -- and I don't have better ideas for solving it either). Unless the null value could be derived from type declaration?

But before merging, I'd suggest one addition that I mentioned, to avoid possibility of "wrong" null value getting cached.
(I forget if caching is done or not, but if it was leaving out access mode could lead to pretty weird failures).