serde_with fails on options that are missing

```rust
#[serde_as]
#[derive(Debug, Deserialize)]
pub struct Foo {
    #[serde_as(deserialize_as = "Option<serde_with::Same>")]
    pub a: Option<u32>,
}

let x = serde_json::from_str("{}");
```

Expected result: deserialized successfully with `a: None`
Actual result: called `Result::unwrap() on an Err value: Error("missing field a")`
