Fix TypeError if task.request.headers is None (#6367)

src/pretix/celery_app.py

@@ -59,7 +59,7 @@ def on_task_prerun(sender, task_id, task, **kwargs):
     from pretix.helpers.logs import local
 
     local.request_id = task_id
-    if "X-Pretix-Trace" in task.request.headers:
+    if task.request.headers and "X-Pretix-Trace" in task.request.headers:
         local.trace = task.request.headers["X-Pretix-Trace"].split(" ")
     else:
         local.trace = []