Hidden file is displayed if there are no visible files

If a project is loaded which has no visible files, but does have a hidden file, then the hidden file is displayed in the file-editor, even though the hidden file is (correctly) not displayed in the tab bar.

Example: https://lit.dev/playground/#project=W3sibmFtZSI6InBhY2thZ2UuanNvbiIsImNvbnRlbnQiOiJ7XG4gIFwiZGVwZW5kZW5jaWVzXCI6IHtcbiAgICBcImxpdFwiOiBcIl4yLjAuMFwiLFxuICAgIFwiQGxpdC9yZWFjdGl2ZS1lbGVtZW50XCI6IFwiXjEuMC4wXCIsXG4gICAgXCJsaXQtZWxlbWVudFwiOiBcIl4zLjAuMFwiLFxuICAgIFwibGl0LWh0bWxcIjogXCJeMi4wLjBcIlxuICB9XG59IiwiaGlkZGVuIjp0cnVlfV0

We should probably have a special screen for the case where there are no visible files (or no files at all), e.g. telling the user to create a new file.