input: Write a function to determine if a given string (str1) is a subsequence of another given string (str2). If str1 is a subsequence of str2, return True; otherwise, return False. If either string is empty, return True if str1 is empty, and False if str2 is empty.
  assert is_subsequence("abc", "qaxbcy")==True
  assert is_subsequence("abc", "qwertyu")==False
  assert is_subsequence("xyz", "xyzxyz")==True
  assert is_subsequence("", "anything")==True
  assert is_subsequence("something", "")==False
output: def is_subsequence(str1, str2):
      if not str1:
          return True
      if 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)