[TTS] Fix text normalizer bugs in TTS data loader
Signed-off-by: Ryan <rlangman@nvidia.com>

# What does this PR do ?

Fixes two bugs recently introduced into the TTS data loader initialization.

1. When pynini is not installed, then Normalizer = None. This causes `isinstance(self.text_normalizer, Normalizer)` to throw an exception.
2. The statement `("text_normalized" not in item) or ("normalized_text" not in item)` should be an `and `rather than an `or`. But rather than make that specific change, I rewrote the condition checks to be less confusing/error prone.

**Collection**:  [TTS]

# Changelog 
- Avoid TypeError when user does not use a text normalizer and pynini is not installed.
- Avoid applying text normalization when "text_normalized" is provided as a field

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

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