Your task is to sum the differences between consecutive pairs in the array in descending order.
For example: sumOfDifferences([2, 1, 10])
Returns 9
Descending order: [10, 2, 1]
Sum: (10 - 2) + (2 - 1) = 8 + 1 = 9
If the array is empty or the array has only one element the result should be 0 (Nothing in Haskell).