### What problem does this PR solve?
API options like `stream` was ignored when no session_id was provided.
This PR fixes the issue.
Test command and expected result:
```
curl --request POST \
--url http://:9222/api/v1/chats/2f2e1d30ee6111efafe211749b004925/completions \
--header 'Content-Type: application/json' \
--header 'Authorization: Bearer ragflow-xxx' \
--data '{
"question":"Who are you",
"stream":false
}'
{"code":0,"data":"data:{\"code\": 0, \"message\": \"\", \"data\": {\"answer\": \"Hi! I'm your assistant, what can I do for you?\", \"reference\": {}, \"audio_binary\": null, \"id\": null, \"session_id\": \"82ceb0fcee7111efafe211749b004925\"}}\n\n"}
```
### Type of change
- [*] Bug Fix (non-breaking change which fixes an issue)
8525f55ad0