zvariant: Struct without fields produce type signature "()" which fails parsing

Hi!

It's my first issue here, so let me start by saying thank you for your work and provided code & documentation! I have only played around with it for a few hours but I am quite pleased with it.

So, I stumbled over a corner case that seems like a bug. Structs without fields (for example struct EmptyStruct {}) can derive zvariant::Type just fine but the resulting signature is '()' which, I believe, is invalid. Or at least the parser can't handle it since this change.

If I revert the linked commit, everything seems to work fine in my case, so maybe reverting it could be a solution. But I don't know why that change was made. And I am really not sure if '()' is a valid signature in the first place, I didn't find it in the specification. So maybe a struct without fields should instead be handled like a unit struct? (Which seems to produce an empty string as signature.)

Anyway, deriving a signature that can't be parsed is not ideal, right? What do you think is the right solution?

Minimal reproducing example:

use zvariant::Type;

#[derive(serde::Serialize, serde::Deserialize, zvariant::Type)]
struct EmptyStruct {}

#[test]
fn test_name() {
    let ctxt = zvariant::EncodingContext::<byteorder::LE>::new_dbus(0);
    let value = EmptyStruct {};
    println!("Signature: '{}'", EmptyStruct::signature());
    let bytes = zvariant::to_bytes(ctxt, &value).expect("couldn't write");
    let _value: EmptyStruct = zvariant::from_slice(&bytes, ctxt).expect("couldn't read");
}

which prints

running 1 test
Signature: '()'
thread 'tests::test_name' panicked at 'couldn't write: Message("invalid length 2, expected >= 2 characters")', src/lib.rs:60:54
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace