Fix: empty api-key causes problems. (#6022)

### What problem does this PR solve?
#5926

### Type of change

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

api/apps/llm_app.py

@@ -135,7 +135,7 @@ def set_api_key():
 def add_llm():
     req = request.json
     factory = req["llm_factory"]
-    api_key = req.get("api_key", "")
+    api_key = req.get("api_key", "x")
     llm_name = req["llm_name"]
 
     def apikey_json(keys):