input: Write a python function to compute the complexity of merge sort for a given array.
  assert merge_complexity([1, 2, 3, 4], 'Merge Sort') == 8.0
  assert merge_complexity([1, 2, 3, 4, 5, 6, 7, 8], 'Merge Sort') == 24.0
  assert merge_complexity([], 'Merge Sort') == 0
output: def merge_complexity(arr, sort_type):
      if sort_type != 'Merge Sort':
          return 0
      n = len(arr)
      if n <= 1:
          return 0
      from math import log2
      return n * log2(n)