Can't navigate test cases in Jenkins

[johnterickson]

https://github.com/johnterickson/cargo2junit/pull/30#issuecomment-668455790

Could you set the classname attribute only when it is available or maybe to a default value like default? Having only src/main.rs and src/lib.rs without further modules there is no path:: prefix in the JSON output. This results in classname="" in the XML output which prevents navigating the test cases in Jenkins.