lint(validators): add missing context

[ee7]

Before this commit, if a track had a JSON file containing a
whitespace-only string, such as:

{
  "files": {
    "solution": [" "],
    "test": ["test_foo.nim"],
    "exemplar": [".meta/exemplar.nim"]
  }
}

the previous output of configlet lint would contain:

    Array contains whitespace-only string: 'solution'

This commit adds the missing context, so that the output of
configlet lint would now contain:

    Array contains whitespace-only string: 'files.solution'

[ee7]

(Ignore the force-push - I pushed to the wrong branch).

[ErikSchierboom]

Great catch