Update app.py

app.py

@@ -13,7 +13,7 @@ LLM = "gpt-4o"
 def invoke(openai_api_key, topic):
     if not openai_api_key:
         raise gr.Error("OpenAI API Key is required.")
-    if not topic == "":
+    if not topic:
         raise gr.Error("Topic is required.")
         
     with lock: