input: Write a function to return the combined length of two given strings. If either of the strings is empty, return the length of the non-empty string. If both strings are empty, return 0.
  assert get_string_length("Hello", "World")==10
  assert get_string_length("", "World")==5
  assert get_string_length("Hello", "")==5
  assert get_string_length("", "")==0
output: def get_string_length(s1, s2):
      if s1 == "" and s2 == "":
          return 0
      elif s1 == "":
          return len(s2)
      elif s2 == "":
          return len(s1)
      else:
          return len(s1) + len(s2)