[TTS] Fix TTS audio preprocessing bugs
# What does this PR do ?

Fix a few bugs in TTS preprocess_audio.py and MelSpectrogramFeaturizer.

**Collection**: [TTS]

# Changelog 
- Add additional config to mel featurizer to make it more consistent with past TTS configs
- Add check and unit test for normalize_volume when audio input is empty array.
- Modify preprocess script to avoid error when resample_rate=0.
- Modify preprocess script to round duration to 2 decimals
- Move normalize_volume unit tests to correct test class, following an earlier refactoring.

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- [x] Make sure you read and followed [Contributor guidelines](https://github.com/NVIDIA/NeMo/blob/main/CONTRIBUTING.md)
- [x] Did you write any new necessary tests?
- [ ] Did you add or update any necessary documentation?
- [ ] Does the PR affect components that are optional to install? (Ex: Numba, Pynini, Apex etc)
  - [ ] Reviewer: Does the PR have correct import guards for all optional libraries?
  
**PR Type**:
- [ ] New Feature
- [x] Bugfix
- [ ] Documentation