Don't nil publication info if article is already published

[Bargraph6]

What does this pull request do?

• app/controllers/admin/articles_controller.rb (Admin::ArticlesController): updated the permitted_params logic for articles to delete nil publication date info if the article is already published.
• spec/requests/article_datetime_settings_spec.rb: Updated spec to allow for separate expected values for the update and create controller actions.

Is there any background context you want to provide for reviewers?

I don't think this is a situation that ever really happens in practice, however while updating the article_datetime_settings_spec, it was discovered that it was technically possible to pass nil to the update action of the articles controller.

This would result in an error further down the line as Article#path would try to strftime the nil:

     NoMethodError:
       undefined method 'strftime' for nil
     Shared Example Group: "handles the publication date" called from ./spec/requests/article_datetime_settings_spec.rb:61
     # ./app/models/article.rb:56:in '
     # ./app/models/article.rb:150:in 'Article#update_or_create_redirect'
     # ./app/controllers/admin/articles_controller.rb:56:in 'Admin::ArticlesController#update'
     # ...

This change just updates the article_params method to remove the published_at and published_at_tz params from the permitted params if the article is already published and the params are nil.

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related_to: #5391