fix: tighten api_key value check in credential validation

litellm/proxy/auth/auth_utils.py

@@ -69,7 +69,8 @@ def check_complete_credentials(request_body: dict) -> bool:
         # complex credentials - easier to make a malicious request
         return False
 
-    if "api_key" in request_body:
+    api_key_value = request_body.get("api_key")
+    if api_key_value and isinstance(api_key_value, str) and api_key_value.strip():
         return True
 
     return False