Remove the generated files by `update-references.sh` if they are empty An empty file may be generated by `update-references.sh` and committed as is when creating a patch like https://github.com/rust-lang/rust-clippy/pull/6101#issuecomment-702076508 and https://github.com/rust-lang/rust-clippy/pull/6079#pullrequestreview-496502721. So, I think it would be helpful to add documentation, and automatically remove the generated file if it's empty. changelog: none