fix(aco)!: do not deserialize empty string to None

[Lencerf]

For a struct like

  struct Foo {
    path: PathBuf,
    param: Option<String>,
  }

to set param to None, it is more intuitive to write path=foo than path=foo,param=. The latter looks more like setting param to Some("").

On the other hand, if the value is an id and the id points to an empty string, it is interpreted as None.

Fixes: 8f3ba3445ac4 ("fix(aco): differentiate Some("") from None")