greptile comment - add None check

Co-authored-by: greptile-apps[bot] <165735046+greptile-apps[bot]@users.noreply.github.com>

Author: parinporecha

posthog/integrations/celery.py

@@ -393,8 +393,11 @@ def _apply_propagated_identity(self, request: Any) -> None:
     def _extract_propagated_tags(self, request: Any) -> dict[str, Any]:
         headers = self._extract_headers(request)
 
+        raw = headers.get(CONTEXT_TAGS_HEADER)
+        if not raw:
+            return {}
         try:
-            parsed = json.loads(headers.get(CONTEXT_TAGS_HEADER))
+            parsed = json.loads(raw)
         except Exception:
             return {}