always set span name in celery

Author: sentrivana

sentry_sdk/integrations/celery/__init__.py

@@ -540,7 +540,7 @@ def sentry_publish(self: "Producer", *args: "Any", **kwargs: "Any") -> "Any":
             # method will still work.
             kwargs_headers = {}
 
-        task_name = kwargs_headers.get("task")
+        task_name = kwargs_headers.get("task") or "unknown celery task"
         task_id = kwargs_headers.get("id")
         retries = kwargs_headers.get("retries")