input: Write a python function to combine two strings character by character.
  assert combine_strings("ab", "xy") == "axby"
  assert combine_strings("abc", "xy") == "axbyc"
  assert combine_strings("ab", "xyz") == "axbyz"
output: def combine_strings(s1, s2):
      combined = []
      for i in range(max(len(s1), len(s2))):
          if i < len(s1):
              combined.append(s1[i])
          if i < len(s2):
              combined.append(s2[i])
      return ''.join(combined)