Error in create_tarred_transformer_lm_dataset.py
**Bug description**

When trying to prepare data for ASR neural LM training, using the script `scripts/asr_language_modeling/neural_rescorer/create_tarred_transformer_lm_dataset.py` getting the following error (and a few preceding log lines):
```
[Parallel(n_jobs=-2)]: Done  68 out of 128 | elapsed:   10.6s remaining:    9.3s
[Parallel(n_jobs=-2)]: Done  81 out of 128 | elapsed:   11.4s remaining:    6.6s
[Parallel(n_jobs=-2)]: Done  94 out of 128 | elapsed:   12.0s remaining:    4.3s
[Parallel(n_jobs=-2)]: Done 107 out of 128 | elapsed:   12.6s remaining:    2.5s
[Parallel(n_jobs=-2)]: Done 120 out of 128 | elapsed:   13.5s remaining:    0.9s
[Parallel(n_jobs=-2)]: Done 128 out of 128 | elapsed:   14.1s finished
Traceback (most recent call last):
  File "./scripts/asr_language_modeling/neural_rescorer/create_tarred_transformer_lm_dataset.py", line 318, in <module>
    main()
  File "./scripts/asr_language_modeling/neural_rescorer/create_tarred_transformer_lm_dataset.py", line 299, in main
    chunk_paths, chunk_lens = __tokenize_text(
  File "./scripts/asr_language_modeling/neural_rescorer/create_tarred_transformer_lm_dataset.py", line 174, in __tokenize_text
    chunk = np.asarray(chunk, dtype=np.int64)
  File "/home/pe.honnet/Tools/miniconda3/envs/nemo/lib/python3.8/site-packages/numpy/core/_asarray.py", line 102, in asarray
    return array(a, dtype, copy=False, order=order)
TypeError: int() argument must be a string, a bytes-like object or a number, not 'NoneType'
Read 1000000 lines: : 1048576 lines [00:18, 57331.13 lines/s]
```

**Steps/Code to reproduce bug**
Command run as follows
```bash
python ./scripts/asr_language_modeling/neural_rescorer/create_tarred_transformer_lm_dataset.py \
  --text_path=training_data.txt \
  --data_root=./data/prep_nn_lm \
  --tokenizer_model=conformer_ctc_large/tokenizer/tokenizer.model \
  --tokenizer_vocab_file=bert_uncased_vocab.txt \
  --tokenizer_name='bert-base-uncased'
```
I have tried using the conformer ctc tokenizer vocab as well with the same error.

**Environment overview**

 - nemo installed with `reinstall.sh` script, on CentOS 8.0
 
**Environment details**

commit used: 034941566 (HEAD -> main, origin/main, origin/HEAD) Merge bugfixes and doc updates from 1.2.0 to main (#2533)
