input: Write a function to read an array of integers and calculate their sum. Return the sum of the array as an integer. If the array is empty, return 0.
  assert calculate_sum([1, 3, 5, 7])==16
  assert calculate_sum([2, 4, 6, 8, 10])==30
  assert calculate_sum([])==0
  assert calculate_sum([0, 0, 0, 0])==0
  assert calculate_sum([-1, 0, 1])==0
output: def calculate_sum(arr):
      if not arr:
          return 0
      return sum(arr)