Allow users to pass nil as a cell and the tablescreen will ignore it.

```ruby
def table_data
  [{
    cells: [
      {title: 'something'},
     nil,
     {title: 'something else'}
    ]
  }]
end
```

will create a tableview with 2 cells instead of throwing an error.

Useful for those of us that want conditional cells based on some other logic and can return nil as a cell.