Struct field with optional child element isn't parsed when child is missing

Consider:
```
<Foo>
    <Bar bing="123">
    </Bar>
    <Bar bing="345">
        <Baz/>
    </Bar>
</Foo>
```

`Bar` has an optional child element `Baz`. It turns out that when this child is missing, as in the first `<Bar>` tag, the element is skipped. This results in the array-valued `Bar` having 1 element, not 2.