lint: allow empty exercises.concept[].concepts

[ee7]

Before this commit, configlet lint would incorrectly exit with a
non-zero exit code if it saw an empty exercises.concept[].concepts
array, even if the corresponding exercises.concept[].status key had
the value deprecated.

Let's use the same temporary workaround for all these arrays, and fix
them all together:

• exercises.concept[].concepts
• exercises.concept[].prerequisites
• exercises.practice[].practices
• exercises.practice[].prerequisites

This commit is a hotfix for the Elixir track CI due to:

• https://github.com/exercism/docs/pull/109/files

---

This PR produces the following diff to the configlet lint output:

elixir

-The `exercises.concept.concepts` array is empty:
-./config.json
-
-Configlet detected at least one problem.
-For more information on resolving the problems, please see the documentation:
-https://github.com/exercism/docs/blob/main/building/configlet/lint.md

ruby

-The `exercises.concept.concepts` array is empty:
-./config.json
-
-The `exercises.concept.concepts` array is empty:
-./config.json
-

[ErikSchierboom]

Thanks for the super quick fix!