Collision between `allow(clippy::needless_lifetimes)` in derive and project workspace `needless_lifetimes = "forbid"`

At this time, in the derive, `allow(clippy::needless_lifetimes)` is used to avoid clippy ranting about the generated lifetime, but is always added even when there are no lifetimes at all.

This clashes with `needless_lifetimes = "forbid"` from the project.

I will make a PR to make sure that these decorators are only added when there are lifetimes.

Luca