fix: add `no-det-res.jpg` to replace 'None' (which cause error when caching examples)

app.py

@@ -104,10 +104,10 @@ def recognize(
         return only_text, str(OUTPUT_RESULT_DIR / out_det_fp)
     elif rec_type == 'formula':
         only_text = p2t.recognize_formula(image_file)
-        return latex_render(only_text), None
+        return latex_render(only_text), './docs/no-det-res.jpg'
     elif rec_type == 'text':
         only_text = p2t.recognize_text(image_file)
-        return only_text, None
+        return only_text, './docs/no-det-res.jpg'
 
 
 def example_func(lang_list, rec_type, image_file):