Fix PTL2.0 related ASR bugs in r1.21.0: Val metrics logging, None dataloader issue
# What does this PR do ?

This PR added fixes for PTL2.0 related ASR bugs in r1.21.0: Val metrics logging, None dataloader issue

**Collection**: 
ASR, Core

# Changelog 
- val_dataloader() and test_dataloader() functions in nemo/core/classes/modelPT.py updated to handle cases where dataloader is None
- correct logging of validation metrics in nemo/collections/asr/models/rnnt_models.py and nemo/collections/asr/models/hybrid_rnnt_ctc_models.py


# 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?
- [ ] 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

# Additional Information
* Related to [Bug4296343](https://nvbugswb.nvidia.com/NvBugs5/SWBug.aspx?bugid=4296343&cmtNo=), [Bug4296057](https://nvbugswb.nvidia.com/NvBugs5/SWBug.aspx?bugid=4296057&cmtNo=), [Bug4246804](https://nvbugswb.nvidia.com/NvBugs5/SWBug.aspx?bugid=4246804&cmtNo=)
* Fixes issues raised in #7450, #7507
