adopt `@IgnoreEmptyDirectories` when Gradle 6.8+ becomes baseline

[nedtwigg]

Just a tracking issue for if/when 6.8 or higher becomes our baseline. Happy to take a PR which uses feature-detection to enable this now with whatever versions we currently support.

https://docs.gradle.org/6.8-rc-1/javadoc/org/gradle/api/tasks/IgnoreEmptyDirectories.html