✨(mailbox) remove secondary email as required field

The secondary email address is no longer required for all creation
processes and we should not force the user to provide and store an
insecure email address.

src/backend/mailbox_manager/api/client/serializers.py

@@ -53,12 +53,19 @@ class MailboxSerializer(serializers.ModelSerializer):
             mailbox.status = enums.MailDomainStatusChoices.ENABLED
             mailbox.save()
 
-            # send confirmation email
-            client.notify_mailbox_creation(
-                recipient=mailbox.secondary_email,
-                mailbox_data=response.json(),
-                issuer=self.context["request"].user,
-            )
+            if mailbox.secondary_email:
+                # send confirmation email
+                client.notify_mailbox_creation(
+                    recipient=mailbox.secondary_email,
+                    mailbox_data=response.json(),
+                    issuer=self.context["request"].user,
+                )
+            else:
+                logger.warning(
+                    "Email notification for %s creation not sent "
+                    "because no secondary email found",
+                    mailbox,
+                )
 
         # actually save mailbox on our database
         return mailbox