community: Fix TypeError in PebbloRetrievalQA (#23170)

**Description:** 
Fix "`TypeError: 'NoneType' object is not iterable`" when the
auth_context is absent in PebbloRetrievalQA. The auth_context is
optional; hence, PebbloRetrievalQA should work without it, but it throws
an error at the moment. This PR fixes that issue.

**Issue:** NA
**Dependencies:** None
**Unit tests:** NA

---------

Co-authored-by: Chester Curme <chester.curme@gmail.com>

libs/community/langchain_community/chains/pebblo_retrieval/base.py

@@ -129,9 +129,7 @@ class PebbloRetrievalQA(Chain):
             "prompt_time": prompt_time,
             "user": auth_context.user_id if auth_context else "unknown",
             "user_identities": auth_context.user_auth
-            if "user_auth" in dict(auth_context)
-            else []
-            if auth_context
+            if auth_context and hasattr(auth_context, "user_auth")
             else [],
         }
         qa_payload = Qa(**qa)