handle path="" correctly in Cargo.toml

[jakecoffman]

It's valid for dependencies in Cargo.toml to point to the package itself with a path of "". So in package jake you could have:

[dev-dependencies]
jake = { path = "" }

This results in Dependabot doing a File.join("", "Cargo.toml") which creates /Cargo.toml instead of Cargo.toml. When it goes to create the file it gets an "Access Denied" error since root isn't writable by the dependabot container user.

This fix trims the leading "/" if present so it correctly returns Cargo.toml.

I considered altering next unless details["path"] to also skip blank paths, but I think the way I've done it here is more correct, as it really is a path dependency and should be returned from path_dependency_paths_from_file. Not sure if that will ever matter though 😃