Fix for gt_ignore (#1242)

If list gt_ignore was empty, it should become None after the following if statement: if not gt_ignore, which was not getting handled correctly. Changed gt_ignore = gt_ignore to gt_ignore = None.

tools/voc_eval.py

@@ -27,7 +27,7 @@ def voc_eval(result_file, dataset, iou_thr=0.5):
         gt_bboxes.append(bboxes)
         gt_labels.append(labels)
     if not gt_ignore:
-        gt_ignore = gt_ignore
+        gt_ignore = None
     if hasattr(dataset, 'year') and dataset.year == 2007:
         dataset_name = 'voc07'
     else: