return well-formatted empty dataframe in case of no vegetation

app.py

@@ -59,6 +59,13 @@ def visualize_instance_seg_mask(img_in, mask, id2label):
         f"{np.sqrt(id2count[id] / image_total_pixels):.2f} m"
         ] for id in label_ids if id2label[id] in vegetation_labels]
 
+    if len(dataframe) < 1:
+        dataframe = [[
+            f"",
+            f"{(0):.2f} %",
+            f"{(0):.2f} m"
+        ]]
+
     return image_res, dataframe