input: Write a function to determine if a given string is a subsequence of another string. If it is a subsequence, return True, otherwise return False. If either of the strings is empty, return False.
  assert is_subsequence("abc", "qaxbcy")==True
  assert is_subsequence("abc", "acb")==False
  assert is_subsequence("abc", "qaxbyc")==False
  assert is_subsequence("abc", "qaxbcyz")==False
  assert is_subsequence("", "qaxbcyz")==False
output: def is_subsequence(s, t):
      if not s or not t:
          return False
      i = 0
      for char in t:
          if i < len(s) and s[i] == char:
              i += 1
      return i == len(s)