fix: allow empty Map/Seq to deserialize as unit struct

[Adjective-Object]

This is part of a series of small fixes bringing serde-content in line with serde's value visitor, to deal with edge-cases.

serde's ContentDeserializer allows empty maps/sequences to deserialize as
unit structs. This is needed for internally tagged enums with unit struct
variants:

#[derive(Deserialize)]
struct Info;

#[derive(Deserialize)]
#[serde(tag = "topic")]
enum Message { Info(Info) }

Where {"topic":"Info"} should deserialize even though unit structs
ordinarily don't deserialize from empty map/seq.

Also changed to fall through to deserialize_any for non-matching cases,
matching serde's behavior.

Reference: https://github.com/serde-rs/serde/blob/d17902059e77e371d8a7f83ff403f9e760b70f45/serde/src/private/de.rs#L1323-L1348