Fix bugs in api (#3624)

### What problem does this PR solve?

#3488

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

---------

Co-authored-by: liuhua <10215101452@stu.ecun.edu.cn>
Co-authored-by: Kevin Hu <kevinhu.sh@gmail.com>

api/apps/sdk/chat.py

@@ -111,7 +111,7 @@ def create(tenant_id):
         req['prompt_config'] = {}
     for key in key_list_2:
         temp = req['prompt_config'].get(key)
-        if not temp:
+        if (not temp and key == 'system') or (key not in req["prompt_config"]):
             req['prompt_config'][key] = default_prompt[key]
     for p in req['prompt_config']["parameters"]:
         if p["optional"]: