PoCiaB nullable 'YEAR' field not tagged as such

The MovieLens sample dataset used in PoC-in-a-Box workshop includes a few movies without years in their title (e.g. ID 162414 *"Moonlight"*).

If I'm not mistaken (I had tweaked the extraction slightly in my fork), the `YEAR` field as extracted by regex has some blanks. However, the item metadata schema in [notebook 2](https://github.com/aws-samples/amazon-personalize-samples/blob/master/next_steps/workshops/POC_in_a_box/02_Validating_and_Importing_Item_Metadata.ipynb) tags:

```json
{
"name": "YEAR",
"type": "int"
}
```

Doesn't it need to be `"type": ["int", "null"]` for this field to be picked up correctly by the model?

Open notebook 02_Validating_and_Importing_Item_Metadata.ipynb and inspect how the YEAR values are extracted before the item metadata schema is defined. Verify whether the blank values require a nullable type, then update the schema if confirmed and ensure the notebook remains consistent with the extracted data.

Written by the indexing model from the issue text.