fix(opentelemetry-instrumentation-celery): attach context only if it …

…is not None

instrumentation/opentelemetry-instrumentation-celery/src/opentelemetry/instrumentation/celery/__init__.py

@@ -170,7 +170,7 @@ def _trace_prerun(self, *args, **kwargs):
         self.update_task_duration_time(task_id)
         request = task.request
         tracectx = extract(request, getter=celery_getter) or None
-        token = context_api.attach(tracectx)
+        token = context_api.attach(tracectx) if tracectx is not None else None
 
         logger.debug("prerun signal start task_id=%s", task_id)