Fix code scanning alert - Potentially uninitialized local variable
when `audio is None`, `z1, log_s_list, log_det_W_list` can not be assigned even `if self.mode == OperationMode.training or self.mode == OperationMode.validation:`.

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Tracking issue for:
- [x] https://github.com/NVIDIA/NeMo/security/code-scanning/905
