input: Write a function to add two large numbers represented as strings. The function should return their sum as a string. If one of the strings is empty then return the other non-empty string as is.
  assert add_large_numbers("111", "1337")=="1448"
  assert add_large_numbers("456", "789")=="1245"
  assert add_large_numbers("", "123")=="123"
  assert add_large_numbers("999", "")=="999"
  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))