Inputs.table skips null values even with custom format callback

In the following example
```js
Inputs.table([{foo: 1, bar: null}], {
  format: {bar: JSON.stringify}
})
```
the format callback is never called, making it impossible to apply custom formatting to nullish values.

Please note that this issue is only concerned with the behavior when a custom format callback is provided. For changes to the default formatter see #164.