Empty XML elements should be represented by empty strings/collections

[RikkiGibson]

A payload like this:

<Root>
    <Child>
        <Item>1</Item>
        <Item>2</Item>
        <Item>3</Item>
    </Child>
</Root>

causes the Root.Child property to be a List instance with 3 items.

<Root>
    <Child />
</Root>

causes the Root.Child property to be null.

Similarly, a missing String property/attribute is represented as null. Objects containing unnecessarily null collections are a pain in the butt to use. We should see if we can make them empty by default instead.

[RikkiGibson]

Note FasterXML/jackson-dataformat-xml#124
There might be some reasonably elegant workaround-- worse case we can change model generation again to accommodate.

[rickle-msft]

Consider support for the case where the child list is not actually included as an element, so
</Root> would still put an non-null empty list in Root.Child even though no child element is even included.