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 the given string is empty, return True.
  assert is_subsequence("abc", "qaxbcy")==True
  assert is_subsequence("axc", "qaxbcy")==True
  assert is_subsequence("acb", "qaxbcy")==False
  assert is_subsequence("xyz", "qaxbcy")==False
  assert is_subsequence("", "qaxbcy")==True
output: def is_subsequence(s, t):
      if not s:
          return True
      i = 0
      for char in t:
          if s[i] == char:
              i += 1
          if i == len(s):
              return True
      return False