Fix __call__ return signature when image is None

app.py

@@ -44,7 +44,7 @@ class App:
 
     def __call__(self, model_name, image):
         if image is None:
-            return ''
+            return '', []
         model = self._get_model(model_name)
         image = self._preprocess(image.convert('RGB')).unsqueeze(0)
         # Greedy decoding