Unable to disable validation
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**Describe the bug**

Setting `model.data.splits_string` to `1000,0,0` results in `TypeError: object of type 'NoneType' has no len()`

**Steps/Code to reproduce bug**

1. Follow the steps in the [GPT model training tutorial](https://docs.nvidia.com/nemo-framework/user-guide/latest/nemotoolkit/nlp/nemo_megatron/gpt/gpt_training.html)
2. Change `model.data.splits_string=\'1000,0,0\'`
3. Run

**Expected behavior**

I should be able to perform pretraining without having to:
1. Provide validation dataset
2. Provide test dataset 
3. Run validation

**Environment overview (please complete the following information)**

N/A. I was able to run the tutorial when `splits_string=\'980,10,10\'`.

**Additional context**

Traceback:
```
Traceback (most recent call last):
  File "/data/projects/11003281/multi-node/cache/source_files/nemo/NeMo/examples/nlp/language_modeling/megatron_gpt_pretraining.py", line 42, in main
    trainer.fit(model)
  File "/data/projects/11003281/multi-node/envs/nemo/lib/python3.10/site-packages/pytorch_lightning/trainer/trainer.py", line 544, in fit
    call._call_and_handle_interrupt(
  File "/data/projects/11003281/multi-node/envs/nemo/lib/python3.10/site-packages/pytorch_lightning/trainer/call.py", line 43, in _call_and_handle_interrupt
    return trainer.strategy.launcher.launch(trainer_fn, *args, trainer=trainer, **kwargs)
  File "/data/projects/11003281/multi-node/envs/nemo/lib/python3.10/site-packages/pytorch_lightning/strategies/launchers/subprocess_script.py", line 105, in launch
    return function(*args, **kwargs)
  File "/data/projects/11003281/multi-node/envs/nemo/lib/python3.10/site-packages/pytorch_lightning/trainer/trainer.py", line 580, in _fit_impl
    self._run(model, ckpt_path=ckpt_path)
  File "/data/projects/11003281/multi-node/envs/nemo/lib/python3.10/site-packages/pytorch_lightning/trainer/trainer.py", line 949, in _run
    call._call_setup_hook(self)  # allow user to set up LightningModule in accelerator environment
  File "/data/projects/11003281/multi-node/envs/nemo/lib/python3.10/site-packages/pytorch_lightning/trainer/call.py", line 94, in _call_setup_hook
    _call_lightning_module_hook(trainer, "setup", stage=fn)
  File "/data/projects/11003281/multi-node/envs/nemo/lib/python3.10/site-packages/pytorch_lightning/trainer/call.py", line 157, in _call_lightning_module_hook
    output = fn(*args, **kwargs)
  File "/data/projects/11003281/multi-node/cache/source_files/nemo/NeMo/nemo/collections/nlp/models/language_modeling/megatron_gpt_model.py", line 1619, in setup
    self.setup_validation_data(self.cfg.data)
  File "/data/projects/11003281/multi-node/cache/source_files/nemo/NeMo/nemo/collections/nlp/models/language_modeling/megatron_gpt_model.py", line 1645, in setup_validation_data
    f'Setting up validation dataloader with len(len(self._validation_ds)): {len(self._validation_ds)} and consumed samples: {consumed_samples}'
TypeError: object of type 'NoneType' has no len()
```