IntentSlotClassificationModel.restore_from fails with TypeError
**Overview**
I am unable to restore an IntentAndSlotClassificationModel from a `.nemo` file using the static `IntentAndSlotClassificationModel.restore_from(model_path)`.

**Steps to reproduce**
These steps rely heavily upon the existing [Joint_Intent_and_Slot_Classification](https://github.com/NVIDIA/NeMo/blob/main/tutorials/nlp/Joint_Intent_and_Slot_Classification.ipynb) tutorial.
1. Prepare training data
    a. Create a temporary directory, /tmp/nlu_evaluation, and two sub-directories: /tmp/nlu_evaluation/conf and /tmp/nlu_evaluation/data.
    b. Download [import_datasets.py](https://raw.githubusercontent.com/NVIDIA/NeMo/v1.2.0/examples/nlp/intent_slot_classification/data/assistant_utils.py) and [import_datasets.py](https://raw.githubusercontent.com/NVIDIA/NeMo/v1.2.0/examples/nlp/intent_slot_classification/data/assistant_utils.py) to /tmp/nlu_evaluation.
    c. Download [master.zip](https://github.com/xliuhw/NLU-Evaluation-Data/archive/master.zip) and extract it in /tmp/nlu_evaluation/data
    d. Process the evaluation data:
        `python /tmp/nlu_evaluation/import_datasets.py --dataset_name=assistant --source_dir=/tmp/nlu_evaluation/NLU-Evaluation-Data-master --target_dir=/tmp/nlu_evaluation/nemo_format`
    e. Download [intent_slot_classification_config.yaml](https://raw.githubusercontent.com/NVIDIA/NeMo/v1.2.0/examples/nlp/intent_slot_classification/conf/intent_slot_classification_config.yaml) to /tmp/nlu_evaluation/conf

2. Train and save the model
Run the following Python code
```
import nemo.collections.nlp as nemo_nlp
from omegaconf import OmegaConf


config = OmegaConf.load('/tmp/nlu_evaluation/conf/intent_slot_classification_config.yaml')
config.model.data_dir = '/tmp/nlu_evaluation/data/nemo_format')
config.trainer.gpus = 1
config.trainer.precision = 16
config.trainer.accelerator = None
config.trainer.max_epochs = 5
trainer = pl.Trainer(**config.trainer)

model = nemo_nlp.models.IntentSlotClassificationModel(config.model, trainer=trainer)
trainer.fit(model)

model.save_to('/tmp/nlu_evaluation/model.nemo')
```

3. Restore the saved model
Run the following Python code
```
import nemo.collections.nlp as nemo_nlp


nemo_nlp.models.IntentSlotClassificationModel.restore_from('/tmp/nlu_evaluation/model.nemo')
```

**Expected behavior**
The model is loaded successfully, without errors or exceptions

**Actual behavior**
The model fails to load because a TypeError is raised.

```
  File ".../lib/python3.8/site-packages/nemo/collections/nlp/models/nlp_model.py", line 429, in restore_from
    return super().restore_from(restore_path, override_config_path, map_location, strict, return_config)
  File ".../lib/python3.8/site-packages/nemo/core/classes/modelPT.py", line 481, in restore_from
    return cls._default_restore_from(restore_path, override_config_path, map_location, strict, return_config)
  File ".../lib/python3.8/site-packages/nemo/core/classes/modelPT.py", line 434, in _default_restore_from
    instance = cls.from_config_dict(config=conf)
  File ".../lib/python3.8/site-packages/nemo/core/classes/common.py", line 472, in from_config_dict
    instance = cls(cfg=config)
  File ".../lib/python3.8/site-packages/nemo/collections/nlp/models/intent_slot_classification/intent_slot_classification_model.py", line 56, in __init__
    self.setup_tokenizer(cfg.tokenizer)
  File ".../lib/python3.8/site-packages/nemo/collections/nlp/models/nlp_model.py", line 111, in setup_tokenizer
    vocab_file = self.register_artifact(config_path='tokenizer.vocab_file', src=cfg.vocab_file)
  File ".../lib/python3.8/site-packages/nemo/collections/nlp/models/nlp_model.py", line 62, in register_artifact
    return super().register_artifact(config_path, src, verify_src_exists=verify_src_exists)
  File ".../lib/python3.8/site-packages/nemo/core/classes/modelPT.py", line 222, in register_artifact
    return_path = os.path.abspath(os.path.join(_NEMO_FILE_FOLDER, src[5:]))
  File ".../lib/python3.8/posixpath.py", line 76, in join
    a = os.fspath(a)
TypeError: expected str, bytes or os.PathLike object, not NoneType
```

**Environment overview (please complete the following information)**
- NeMo version: 1.2.0
- Environment location: bare metal
- Method of NeMo install: pip

**Environment details**
- OS: Ubuntu 20.04
- PyTorch 1.8.1
- Python 3.8
