Allow enum variants to be serialized as empty tags

[leftmostcat]

This brings enum variants in line with structs, wherein structures which have no fields which are represented as child nodes are serialized as empty tags. This is, generally-speaking, cosmetic only.

An example enum:

enum Foo {
    Bar {
        #[xml_struct(attribute)]
        baz: &'static str,
    }
}

Prior to this change, Foo::Bar { baz: "qux" } would be serialized as follows:

<Bar Baz="qux"></Bar>

Following this change, it will be serialized as follows:

<Bar Baz="qux"/>

[babolivier]

lgtm