input: Write a function to add two large numbers represented as strings. If either of the numbers is an empty string, return the other number.
  assert add_large_numbers("111", "1337")=="1448"
  assert add_large_numbers("123456789", "987654321")=="1111111110"
  assert add_large_numbers("", "123")=="123"
  assert add_large_numbers("456", "")=="456"
  assert add_large_numbers("", "")==""
output: def add_large_numbers(num1, num2):
      if not num1:
          return num2
      if not num2:
          return num1
      return str(int(num1) + int(num2))