input: Write a function to calculate the sum of an array of integers. If the given array is empty, return 0.
  assert calculate_sum([1, 3, 5, 7])==16
  assert calculate_sum([10, 20, 30])==60
  assert calculate_sum([-1, 0, 1])==0
  assert calculate_sum([])==0
output: def calculate_sum(arr):
      if not arr:
          return 0
      return sum(arr)