Fix deserialization of null values for collection/array type deserializers
* All of these deserializers should ask the encapsulated valueDeserializer for its nullValue when `VALUE_NULL` is encountered, rather than assuming a return value of `null`.
* This is a bug that affects the ser/deser of `Collection[Option[_]]` in scala, as `None` values are serialized to `null` and deserialized back to `null` rather than `None`, which is correctly overriden in `OptionDeserializer`
* Note I am a first time committer and happy to sign any release. Thanks!
tests for custom null deser with collections
unit tests showing custom null deserialization of values in arrays and maps
Merge pull request #407 from wpalmeri/null-deser-cleanup

Fix deser of null values for collection/array type deserializers