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.