fix: streaming_query: empty conversation ID

The query endpoint would treat an empty conversation ID as not provided,
but the streaming query endpoint would treat it as if it were an ID
(which doesn't belong to anyone, so it would fail).

Align the behavior of the streaming query endpoint with the query
endpoint, so when users provide a "conversation_id" with an empty
string, it will be as if they didn't provide it at all.

Author: omertuc

src/app/endpoints/streaming_query.py

@@ -399,7 +399,7 @@ async def streaming_query_endpoint_handler(  # pylint: disable=too-many-locals
     user_id, _user_name, token = auth
 
     user_conversation: UserConversation | None = None
-    if query_request.conversation_id is not None:
+    if query_request.conversation_id:
         user_conversation = validate_conversation_ownership(
             user_id=user_id, conversation_id=query_request.conversation_id
         )