Adjacently tagged enum does not respect skip_serializing_if

[davidMcneil]

Here is an example showing the actual and expected output:

use serde::{Deserialize, Serialize};

#[derive(Deserialize, Serialize)]
#[serde(tag = "key", content = "value")]
enum MyEnum {
    A(#[serde(skip_serializing_if = "Option::is_none")] Option<u32>),
}

#[test]
fn it_works() {
    let a = MyEnum::A(None);
    // The actual output
    assert_eq!(
        "{\"key\":\"A\",\"value\":null}",
        serde_json::to_string(&a).unwrap()
    );
    // The expected output
    assert_eq!("{\"key\":\"A\"}", serde_json::to_string(&a).unwrap());
}

[Mingun]

Internally this is handled by the same code as the one in #2105, so technically one is duplicate of another