Empty OID crashes the cargo doumentation generation with oid feature

[mrnossiom]

👋🏻

While building a project with the pgp lib, I could not build the documentation due to failed evaluation of constant value. Such empty OID generations would make const-oid panic due to how ObjectIdentifier::new_unwrap is made. I don't think this is removing any feature, since it would panic during runtime.

I took the opportunity to fix a rustdoc lint.

[mrnossiom]

For sure