input: Write a function to add two large numbers represented as strings. The function should return the sum of the two numbers also as a string. If the input strings are empty then return an empty string.
  assert add_large_numbers("111", "1337")=="1448"
  assert add_large_numbers("123", "456")=="579"
  assert add_large_numbers("", "1337")=="1337"
  assert add_large_numbers("111", "")=="111"
  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))