Fix failure of ljspeech's get_data.py
# What does this PR do?

When running `python scripts/dataset_processing/tts/ljspeech/get_data.py --data-root <your_local_dataset_root>` to get the data of LJSpeech as the [document](https://docs.nvidia.com/deeplearning/nemo/user-guide/docs/en/main/tts/datasets.html#ljspeech), it reported error:
```
FileNotFoundError: [Errno 2] No such file or directory: 'lj_speech.tsv extracted from the readme file in the dataset. You can also download the file from https://github.com/NVIDIA/NeMo-text-processing/blob/main/nemo_text_processing/text_normalization/en/data/whitelist/lj_speech.tsv'
```
Solution: The default value of argument `--whitelist-path` of `get_data.py` should be `None` (so the get_data.py will automatically download the lj_speech.tsv) instead of a long string.

# Changelog 
- Fix failure of ljspeech's get_data.py.

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**PR Type**:
- [ ] New Feature
- [x] Bugfix
- [ ] Documentation

## Who can review?

@blisc @okuchaiev @titu1994