Fix missing body and TypeError when queuing system mails

Problem/Motivation

When using the Symfony Mailer module, system emails such as user/password_reset are queued without a subject or body.
This happens because the SymfonyQueueMailAdjuster runs before the core adjusters have populated the Email object.

Additionally, the body can be stored as an array which is later passed to check_markup(), causing a TypeError.

Steps to reproduce

• Enable symfony_mailer and set the Mail Entity Queue with the Symfony Mailer processor.
• Configure the queue to capture outgoing system emails using the EmailAdjuster.
• Trigger a password reset request.
• Observe that the created mail_entity_queue_item entity has empty subject and body.
• When processing the queue, the following error occurs:
  

  TypeError: stristr(): Argument #1 ($haystack) must be of type string, array given
  in Drupal\editor\Plugin\Filter\EditorFileReference->process() (core/modules/editor/src/Plugin/Filter/EditorFileReference.php line 83)