Handle Authors field being optional in Rust v1.53.0 or newer

[volks73]

As of Rust v1.53.0, the authors field is optional and not automatically added to the project's manifest, Cargo.toml. The current minimum version supported by this project is v1.40.0 and the tests currently run with Rust v1.50.0. However, if the tests are run with a newer version of Rust, >v1.53.0, the integration tests will fail with a Manifest("authors") error.

The test fixtures assume the authors field exist because cargo originally always added the field and it was required. The field is now optional, but it is used for the manufacturer and other metadata of the installer. A backwards compatible fix needs to be added and the assumption changed. The authors field should be treated like the other optional fields, i.e. a warning if not defined.

References

1. https://stackoverflow.com/questions/67891784/how-can-i-set-default-authors-for-new-cargo-projects
2. https://github.com/rust-lang/rfcs/blob/master/text/3052-optional-authors-field.md