Fix empty string in some messages

main.py

@@ -28,7 +28,7 @@ app.state.auto_polling = False
 app.state.polling_rate = 5
 app.state.enable_libretranslate = True
 app.state.client_state = "idle"
-app.state.latest_message = ""
+app.state.latest_message = None
 
 # Define the media directory
 MEDIA_DIR = Path("./media")
@@ -130,6 +130,7 @@ async def control_endpoint(request: Request):
         
         if data['command'] == "getselectedmessage":
             message = await fetch_selected_message()
+            message = None if message == "" else message
             await process_selected_message(message)
             logger.info(f"Received new selected message initiated from control: {message}")