[Config] fixes broken unit test on ArrayNode class.
minor #13050 [Config] fixes broken unit test on ArrayNode class. (hhamon)

This PR was merged into the 2.3 branch.

Discussion
----------

[Config] fixes broken unit test on ArrayNode class.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | ~
| License       | MIT
| Doc PR        | ~

Commits
-------

a7ee58a [Config] fixes broken unit test on ArrayNode class.