Add new feature, `FromXmlParser.Feature.EMPTY_ELEMENT_AS_NULL`

[cowtowncoder]

Since there are two possible ways to expose XML empty element (like "" -- but not ""), it seems reasonable to allow configuration of how this is to be exposed.
Due to backwards compatibility reasons, setting should default to true, so that by default empty element would be exposed as equivalent of JSON null token (JsonToken.VALUE_NULL).
But disabling of this feature would instead expose it as JsonToken.VALUE_STRING with text value of "" (empty String).

[anton-johansson]

I'm really looking forward to this feature. Is there any ETA on 2.9.0?

I see a bunch of pre-releases here:
http://mvnrepository.com/artifact/com.fasterxml.jackson.dataformat/jackson-dataformat-xml

But none of them seem to include this commit.

[cowtowncoder]

I have been hoping to finalize 2.9.0 for a while now; 2.9.0.pr4 is meant as the last pre-release version.
I am now hoping to get 2.9.0 out by end of July 2017.

[anton-johansson]

Cool, thanks for the update!