Error when assign style to Text
The following code throw an error of `Error: image needs to be a Buffer`, but:

1. The fill is not enabled.
2. even it is enabled, the fill type is color, not pattern
3. `null` is valid to `pattern.image` according to api document

So, it's I think it's a bug of the api environment.

```ts
let subText = new sketch.Text({
    text: '123',
    parent: sketch.getSelectedDocument().pages[0].layers[0],
});
subText.style = {
    fills: [
        {
            enabled: false,
            fillType: sketch.Style.FillType.Color,
            pattern: {
                image: null
            }
        }
    ]
}
```