allowable nullable fields

it woudl be nice to handle nullable fields and output something like
```py 
Union[int,None]
```

This would require checking ocurrences of a field across an array of objects e.g.

```json
[
    {
         "foo": "bar",
         "baz": 1
    },
    {
         "foo": "bar"
    }
]

```

which should output

```
class GeneratedClass:
    foo: str
    baz: Union[int,None]
```