input: Write a function to read an array of integers and calculate their sum. If the array is empty, return 0.
  def calculate_sum(arr):
      if not arr:
          return 0
      sum = 0
      for i in arr:
          sum += i
      return sum
  
  assert calculate_sum([1, 3, 5, 7])==16
  assert calculate_sum([2, 4, 6, 8])==20
  assert calculate_sum([0, 0, 0, 0])==0
  assert calculate_sum([])==0
output: def calculate_sum(arr):
      if not arr:
          return 0
      sum = 0
      for i in arr:
          sum += i
      return sum