rails: config_for raises an error for arrays when there's only a shared key

Steps to reproduce

I’m using a config files to blacklist some jwt tokens like this:

# config/jwt_blacklist.yml
---
shared:
  - token1
  - token2

In application.rb

config.jwt_whitelist = config_for(:jwt_whitelist)

Expected behavior

Rails.configuration.jwt_whitelist
=> ["token1", "token2"]

Actual behavior

railties-6.1.4/lib/rails/application.rb:252:in `config_for': undefined method `deep_merge' for ["token1", "token2"]:Array (NoMethodError)

It worked on rails 6.1.3.2, I think it has been introduced with https://github.com/rails/rails/commit/3fe0ab52dff018ed940ad45885c9d90208eb3cac (related issue: https://github.com/rails/rails/issues/42037 )

Thanks to this issue I think it should accepts arrays.

Moreover, I tried the following:

production: []
test: []
development: []
shared:
  - token1
  - token2

And it returns an empty array:

Rails.configuration.jwt_whitelist
=> []

Not sure if it’s a normal behaviour too.

System configuration

Rails version: 6.1.4

Ruby version: 2.7.2p137