This affects performance of the serde deserializer by ~20%, unfortunately

failures (11):
    as_field::empty::nested::true_
    as_field::with_attr::nested::true_
    as_field::with_element::nested::true_
    as_field::with_element::ns0::true_
    as_field::with_element::xsi::true_
    top_level_option::empty::ns0::true_
    top_level_option::empty::xsi::true_
    top_level_option::with_attr::ns0::true_
    top_level_option::with_attr::xsi::true_
    top_level_option::with_element::ns0::true_
    top_level_option::with_element::xsi::true_

This commit fixes an issue that causes `quick-xml` trying to deserialize
empty tags via the serde interface even if these tags were explicitly
marked as `xsi:nil="true"`

For example the following XML failed to deserialize before this commit:

```xml
<bar>
	<foo xsi:nil="true"/>
</bar>
```

into the following rust type:

```rust
#[derive(Deserialize)]
struct Bar {
    foo: Option<Foo>,
}

#[derive(Deserialize)]
struct Foo {
    baz: String,
}
```

Before this commit this failed to deserialize with an error message that
complained that the `baz` field was missing. After this commit this uses
the `xsi:nil` attribute to deserialize this into `foo: None` instead.
The standard (https://www.w3.org/TR/xmlschema-1/#xsi_nil) seems to
support this behaviour.

Fix #497

Properly use xsi:nil to deserialize null values via serde