feat(input_schema): Allow nullable field default to null (#523)

Solves apify/apify-core#20247

The `default` property should allow `default: null` if field has
`nullable: true`

```
"test": {
    "title": "test",
    "description": "test",
    "type": "integer",
    "nullable": true, 
    "default": null
}
```

Thanks to `unevaluatedProperties` from draft 2019 it's quite easy now

Author: mfori