lint(validators): support empty array as value of required key (#262)
lint(validators): support empty array as value of required key (#262)

There are some keys for which we want to print an error if the key is
missing, but not if the key has a value that is the empty array. This
commit gives us a way to express that.

Some relevant keys in the track `config.json` file:
- `exercises.concept` (many tracks only have Practice Exercises)
- `exercises.practice` (theoretically, a future track could have only
  Concept Exercises)
- `concepts` (many tracks do not have Concepts)

exercism/configlet