Support empty transcription in metric computation

EPOCH 0/800:  92%|▉| 328/356 [00:53<00:04,  6.08it/s, values={'loss_ce': 4.9419, 'cer': 0.9445, 'cer_no_token': 0.9884, 'wer': 1.0012, 'we
Traceback (most recent call last):
  File "/home/users/ebardou/.virtualenvs/dan/bin/teklia-dan", line 8, in <module>
    sys.exit(main())
  File "/home/users/ebardou/.virtualenvs/dan/lib/python3.10/site-packages/dan/cli.py", line 31, in main
    status = args.pop("func")(**args)
  File "/home/users/ebardou/.virtualenvs/dan/lib/python3.10/site-packages/dan/ocr/train.py", line 127, in run
    start_training(config, mlflow_logging=False)
  File "/home/users/ebardou/.virtualenvs/dan/lib/python3.10/site-packages/dan/ocr/train.py", line 103, in start_training
    train(0, config, mlflow_logging)
  File "/home/users/ebardou/.virtualenvs/dan/lib/python3.10/site-packages/dan/ocr/train.py", line 50, in train
    model.train(mlflow_logging=mlflow_logging)
  File "/home/users/ebardou/.virtualenvs/dan/lib/python3.10/site-packages/dan/ocr/manager/training.py", line 672, in train
    batch_metrics = self.metric_manager["train"].compute_metrics(
  File "/home/users/ebardou/.virtualenvs/dan/lib/python3.10/site-packages/dan/ocr/manager/metrics.py", line 207, in compute_metrics
    metrics[keyword + "_error_rate" + suffix] = [
  File "/home/users/ebardou/.virtualenvs/dan/lib/python3.10/site-packages/dan/ocr/manager/metrics.py", line 208, in <listcomp>
    round(float(edit_dist / gt_len), 4)
ZeroDivisionError: division by zero