input: Write a function to determine if a given string (str1) is a subsequence of another given string (str2). If str1 is a subsequence, return True; otherwise, return False. If either string is empty, return False.
  assert is_subsequence("abc", "qaxbcy")==True
  assert is_subsequence("abc", "acb")==False
  assert is_subsequence("xyz", "xzyw")==False
  assert is_subsequence("abc", "")==False
  assert is_subsequence("", "abc")==False
output: if not str1 or not str2:
      return False
  i, j = 0, 0
  while i < len(str1) and j < len(str2):
      if str1[i] == str2[j]:
          i += 1
      j += 1
  return i == len(str1)