define `average` for empty lists

[adampingel]

https://github.com/randomforestwhitaker/questions/blob/a607680fed8c614a478260e329b4412a57112903/answer_1.scala#L43-L45

This helper function would be more helpful if it were defined on empty lists. Check the incoming list's length. If it's zero, return Double.NaN.

This would allow you to remove the check for Nil in resolver above.

[randomforestwhitaker]

Moved empty list handling logic to this average helper function instead of main resolver function.