Fatal error when registering for event without entering email

Problem/Motivation

When registering for an event, none of the fields in the RegistrantForm are required. When the form is submitted without entering an email address, a fatal error occurs.

Steps to reproduce

1. Create an event series.
2. Enable registration.
3. Go to one of the event instances and click "Register".
4. Don't fill in the email address, just scroll down and press "Register" again.

Result:

TypeError: Drupal\Core\Mail\Plugin\Mail\PhpMail::doMail(): Argument #1 ($to) must be of type string, null given, called in /var/www/html/core/lib/Drupal/Core/Mail/Plugin/Mail/PhpMail.php on line 123 in Drupal\Core\Mail\Plugin\Mail\PhpMail->doMail() (line 168 of core/lib/Drupal/Core/Mail/Plugin/Mail/PhpMail.php).

Drupal\Core\Mail\Plugin\Mail\PhpMail->mail() (Line: 308)
Drupal\Core\Mail\MailManager->doMail() (Line: 181)
Drupal\Core\Mail\MailManager->Drupal\Core\Mail\{closure}() (Line: 638)
Drupal\Core\Render\Renderer->executeInRenderContext() (Line: 180)
Drupal\Core\Mail\MailManager->mail() (Line: 426)
recurring_events_registration_send_notification() (Line: 96)
Drupal\recurring_events_registration\Entity\Registrant->postSave() (Line: 563)
Drupal\Core\Entity\EntityStorageBase->doPostSave() (Line: 781)
Drupal\Core\Entity\ContentEntityStorageBase->doPostSave() (Line: 489)
Drupal\Core\Entity\EntityStorageBase->save() (Line: 806)
Drupal\Core\Entity\Sql\SqlContentEntityStorage->save() (Line: 354)
Drupal\Core\Entity\EntityBase->save() (Line: 293)
Drupal\Core\Entity\EntityForm->save() (Line: 489)
Drupal\recurring_events_registration\Form\RegistrantForm->save()
call_user_func_array() (Line: 129)

Proposed resolution

Make the email base field required in Registrant::baseFieldDefinitions().