pom.xml: add link to jackson-annotations javadoc
Add a custom deserializer for JsonNode, to avoid nulls

The default JsonNodeDeserializer implementation returns Java null when the next
token is JsonToken.VALUE_NULL.

But this is not what we want here: we want a NullNode instead. Extend
JsonNodeDeserializer an make it return a NullNode when the next token is null,
and annotate PathValueOperation appropriately.