Envelope deserialization doesn't catch empty forward_path

[wyfo]

Describe the bug
Envelope deserialization doesn't catch empty forward_path.

To Reproduce

serde_json::from_str::<Envelope>(r#"{"forward_path": []}"#).unwrap() // should panic, but don't

Expected behavior
Deserializing envelope with empty forward_path should panic.

Environment (please complete the following information):

• Lettre version: 0.10.0
• OS: Ubuntu

Additional context
I think I will open a PR to fix it soon (maybe tonight)

[Agreon]

@paolobarbolini would you consider adding a custom type like in https://serde.rs/custom-date-format.html to check for the vec length as a valid fix?