add conditional logic to rnnt_wer to handle when arrays have no elements
# What does this PR do ?

Adds conditional logic to rnnt_wer to check if arrays exist and have elements before accessing them. This error occasionally blocks transcription.


**Collection**:  ASR

# Changelog 

Bug fix for transcription with rnnt sentencepiece tokens


# Before your PR is "Ready for review"
**Pre checks**:
- [X] Make sure you read and followed [Contributor guidelines](https://github.com/NVIDIA/NeMo/blob/main/CONTRIBUTING.md)

~- [ ] Did you write any new necessary tests?~ Unable to share specific test file that found this error
- [X] 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
