Handle "" properly in ListConverter.

When an empty string is passed it should return an empty list rather than a list of one empty string.

I believe that the proposed behaviour is more useful than the current. The converter can be used to easily specify a variable number of arguments in a column. Users most likely want to be able to specify no elements at all.

Does this change sound reasonable ? 

PS: I decided to return a new ArralyList rather than an empty collection from Collections because I expect that some users mutate the list in their Step def.