Serializing nulls and empty Strings in dynamic JSON
There are things you might want to try.
First, try using @XmlElement(nillable=true) instead of XmlNullPolicy annotation or setting the emptyNodeRepresentsNull parameter.
Second, you might want to write your own XmlAdapter which will do pretty much the same thing - unmarshall empty string to null. The main difference is that you can also manually configure which fields will be unmarshalled with your adapter and which will not, therefore retaining the current behaviour. Please refer to this answer by Blaise Doughan to see how do you wire a custom XmlAdapter to your configuration.
Third, as far as I know Jackson doesn't have this problem. See this answer ot this wiki page about how to set up a custom deserializer for a field in case it does.