Fix #4518: Tell MiMa not to fail on empty mimaPreviousArtifacts.
Fix #4518: Tell MiMa not to fail on empty mimaPreviousArtifacts.

sjrd/scala-js,sjrd/scala-js,scala-js/scala-js,gzm0/scala-js,scala-js/scala-js,gzm0/scala-js,sjrd/scala-js,scala-js/scala-js