Fix defer_setup issue of failing to create validation and test dataloaders
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# What does this PR do ?

After #7455 , `ModelPT.val_dataloader()` and `ModelPT.test_dataloader()` return empty lists instead of None.
This modification causes `ModelPT.setup()` failed to pass the 'if' statements at line 835 and 844, and the dataloaders are therefore remaining empty lists without construction:
https://github.com/NVIDIA/NeMo/blob/8216178cfaf89228884bd1e4205776a2667c6e51/nemo/core/classes/modelPT.py#L829-L845

This PR changes the `self.xxx_dataloader() is None` statements into `not self.xxx_dataloader()`, such that both "None" and empty lists could fit the condition.


**Collection**: All

# Changelog 
- change `self.xxx_dataloader() is None` into `not self.xxx_dataloader()`, so as to handle both None and empty lists
```
def setup(self, stage: Optional[str] = None):
    ...

    if stage in ('fit', 'validate'):
        val_deferred_setup = ...
        if not self.val_dataloader() and val_deferred_setup:
            self.setup_multiple_validation_data(val_data_config=self._cfg.validation_ds)

    if stage == 'test':
        test_deferred_setup = ...
        if not self.test_dataloader() and test_deferred_setup:
            self.setup_multiple_test_data(test_data_config=self._cfg.test_ds)
```

# 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 #5569 , #7455 
