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 the first string is empty then return True.
  assert is_subsequence("abc", "qaxbcy")==True
  assert is_subsequence("abc", "qawxbyc")==False
  assert is_subsequence("hello", "hello world")==True
  assert is_subsequence("sing", "sting")==True
  assert is_subsequence("", "abc")==True
output: def is_subsequence(s1, s2):
      if not s1:
          return True
      i = 0
      for char in s2:
          if i < len(s1) and s1[i] == char:
              i += 1
          if i == len(s1):
              return True
      return False