Add check for '' instead.

app.py

@@ -88,7 +88,7 @@ def generate_for_prompt(input_text, state, ret_scale_factor, num_words, temperat
     model_inputs = chat_history
     model_inputs.append(input_prompt)
     # Remove empty text.
-    model_inputs = [s for s in model_inputs if len(s) > 0]
+    model_inputs = [s for s in model_inputs if s != '']
 
     top_p = 1.0
     if temperature != 0.0: