ShortObjectCoerce returns null for empty values instead of zero 

I made this change when I had an output object that had Doubles, Longs, Shorts and Integer objects with values not set. For all types other than Short the output from Cascading was empty (which is what I want), but for Shorts it was zero. I had a look at all the Coercion types for Number objects and noticed ShortObjectCoerce was the only one that output a zero in this case - all the others output null. So I have changed ShortObjectCoerce to output null too. There weren't tests for the Coercion classes so I added one for ShortObjectCoerce to test my change and then added one for IntegerObjectCoerce for comparison.