allow parsing empty array into unit

[jsen-]

While working on JS <-> rust interop crate, I'm trying to serialize function arguments on the javascript side and deserialize into a tuple with appropriate number of elements on the rust side. Unfortunately I found that it doesn't work for functions with no arguments.

serde_json::from_str::<(u8,u8)>("[1, 2]").unwrap(); // is fine
serde_json::from_str::<(u8,)>("[1]").unwrap(); // is fine
serde_json::from_str::<()>("[]").unwrap(); // currently fails

This PR allows the last line to pass.

[andogq]

I would also be interested in this feature being merged! I've run into the exact same use-case. Happy to assist with any contributions required to get this merged! :)

If anyone else is facing this issue, [(); 0] works as a stand-in for () and will happily deserialise from []: https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=91e7fd458c369b2dd74e13d90ae2f03d