Fix handling of blank form fields.

If an `ArrayFormField` is submitted empty, it comes into the `clean` method as an empty string. Before this fix, it would be split on the delimiter, resulting in a length-one list containing the empty string. For an array field holding integers, this would cause an error in `_cast_to_type` when trying to cast an empty string to an integer.

Instead, an empty value should be cleaned to the empty list.