feat: participant's email address mandatory (again)

closes #86991wmpc

src/Form/Model/ParticipantData.php

@@ -32,6 +32,7 @@ class ParticipantData
     #[Assert\NotNull(message: 'Bitte angeben')]
     public ?\DateTimeImmutable $dateOfBirth = null;
 
+    #[Assert\NotBlank(message: 'Bitte angeben')]
     #[Assert\Email(message: 'Bitte eine gültige E-Mail Adresse angeben', mode: 'strict')]
     public ?string $email = null;