Empty string results in error

I have a field defined like this:

```
    #[serde(default, deserialize_with = "deserialize_option_duration")]
    pub runtime: Option<Duration>,
```

but when the json string is empty:

`"runtime":""`

deserialising will fail:

 Error("Empty input", line: 1, column: 1678)

I would expect it to set it to None and not fail