input: Write a function to create a Python class that takes a given number and calculates the sum of its digits. If the given number is empty then return 0.
  class SumOfDigits:
      
      def __init__(self, num):
          self.num = num
      
      def calculate_sum_of_digits(self):
          if self.num == "":
              return 0
          total = 0
          for ch in str(self.num):
              total += int(ch)
          return total
  
  assert SumOfDigits("6342").calculate_sum_of_digits() == 15
  assert SumOfDigits("12345").calculate_sum_of_digits() == 15
  assert SumOfDigits("").calculate_sum_of_digits() == 0
  assert SumOfDigits("987654321").calculate_sum_of_digits() == 45
  assert SumOfDigits("0").calculate_sum_of_digits() == 0
output: def __init__(self, num):
      self.num = num
  
  def calculate_sum_of_digits(self):
      if self.num == "":
          return 0
      total = 0
      for ch in str(self.num):
          total += int(ch)
      return total