input: Write a function to determine if one string is a subsequence of another string. The function should return True if the first string is a subsequence of the second string, and False otherwise. If either of the strings is empty, the function should return False.
  assert is_subsequence("abc", "qaxbcy")==True
  assert is_subsequence("axc", "ahbgdc")==False
  assert is_subsequence("abc", "abc")==True
  assert is_subsequence("", "ahbgdc")==False
  assert is_subsequence("abc", "")==False
output: def is_subsequence(s, t):
      if not s:
          return False
      if not t:
          return False
      i, j = 0, 0
      while i < len(s) and j < len(t):
          if s[i] == t[j]:
              i += 1
          j += 1
      return i == len(s)