Fix mail headers being None (#5873)

* Fix mail headers being None

* update tests

src/pretix/base/services/mail.py

@@ -321,7 +321,7 @@ def mail(email: Union[str, Sequence[str]], subject: str, template: Union[str, La
             body_plain=body_plain,
             body_html=body_html,
             sender=sender,
-            headers=headers,
+            headers=headers or {},
             should_attach_tickets=attach_tickets,
             should_attach_ical=attach_ical,
             should_attach_other_files=attach_other_files or [],
@@ -763,7 +763,7 @@ def mail_send(to: List[str], subject: str, body: str, html: Optional[str], sende
         body_plain=body,
         body_html=html,
         sender=sender,
-        headers=headers,
+        headers=headers or {},
         should_attach_tickets=attach_tickets,
         should_attach_ical=attach_ical,
         should_attach_other_files=attach_other_files or [],

src/tests/base/test_mail.py

@@ -170,7 +170,6 @@ def test_queue_state_sent(env):
         subject='Test',
         body_plain='Test',
         sender='sender@example.com',
-        headers={},
     )
     assert m.status == OutgoingMail.STATUS_QUEUED
     mail_send_task.apply(kwargs={
@@ -188,7 +187,6 @@ def test_queue_state_permanent_failure(env):
         subject='Test',
         body_plain='Test',
         sender='sender@example.com',
-        headers={},
     )
     assert m.status == OutgoingMail.STATUS_QUEUED
     mail_send_task.apply(kwargs={
@@ -210,7 +208,6 @@ def test_queue_state_retry_failure(env, monkeypatch):
         subject='Test',
         body_plain='Test',
         sender='sender@example.com',
-        headers={},
     )
     assert m.status == OutgoingMail.STATUS_QUEUED
     mail_send_task.apply(kwargs={
@@ -236,7 +233,6 @@ def test_queue_state_foreign_key_handling():
         subject='Test',
         body_plain='Test',
         sender='sender@example.com',
-        headers={},
     )
     mail_sent = OutgoingMail.objects.create(
         organizer=o,
@@ -245,7 +241,6 @@ def test_queue_state_foreign_key_handling():
         subject='Test',
         body_plain='Test',
         sender='sender@example.com',
-        headers={},
         status=OutgoingMail.STATUS_SENT,
     )