Do not call hook_scheduler_hide_publish_on_field if the field is already hidden

In #2798689: Scheduler integration with core Content moderation I added the code to call each implementation of hook_scheduler_hide_publish_on_field to check whether the publish_on field should be hidden in the scheduler node form.

  // Invoke all implementations of hook_scheduler_hide_publish_on_field() and
  // hook_scheduler_hide_unpublish_on_field() to allow other modules to hide the
  // fields on the node edit form.
  $hook = 'scheduler_hide_publish_on_field';
  foreach (\Drupal::moduleHandler()->getImplementations($hook) as $module) {
    $function = $module . '_' . $hook;
    $publishing_enabled = $publishing_enabled && ($function($form, $form_state, $node) !== TRUE);
  }

However in #3061630: Undefined index: publish_state when publish field is not present the hook implementation causes an error if the field is not in the form, i.e. already hidden. This should probably be fixed in Scheduler by only running the above foreach when $publishing_displayed is true.

Minimum steps to re-produce:

1. Clean install of core 8.7
2. Enable Content Moderation, Scheduler and SCMI modules
3. Config - Workflows - Edit 'Editorial' - 'This workflow applies to content type' - select Article
4. Enable Article content type for scheduled publishing
5. In Article 'manage form display' disable both 'Publish On' and 'Publish State' fields
6. In /admin/config/development/logging select 'All messages'
7. Create Article content