input: Write a function to determine if the first string is a subsequence of the second string. If it is a subsequence return True else return False. If either string is empty then return False.
  assert is_subsequence("abc", "qaxbcy")==True
  assert is_subsequence("abc", "qaxcy")==False
  assert is_subsequence("abc", "")==False
  assert is_subsequence("", "qaxbcy")==False
  assert is_subsequence("", "")==False
output: def is_subsequence(s1, s2):
      if not s1 or not s2:
          return False
      i = 0
      for char in s2:
          if i < len(s1) and s1[i] == char:
              i += 1
      return i == len(s1)