.map() does not behave like jQuery.map()

jQuery's map implementation allows for the removal of an item from the resulting array by returning either null or undefined.

Cheerio's implementation leaves an empty placeholder.

From jQuery's documentation:

> The function can return:
> 
> the translated value, which will be mapped to the resulting array
> null or undefined, to remove the item
> an array of values, which will be flattened into the full array
>