Topic/1 fail empty list

This PR fixes problem with `Cannot access first() element from an empty List` (#1) which can be caused by an empty found artifacts list returned by Maven Central for  artifacts from other repositories (like jcentral). In addition DefaultSelfResolvingDependency internally added by Gradle are ignored and not displayed as `null:unspecified:null`.
By the way header is not displayed when no new versions have been found and some simplifications in test code were introduced.