input: Write a function to determine if a given string (str1) is a subsequence of another string (str2). If str1 is a subsequence, return True, else return False. If either str1 or str2 is an empty string, return False.
  assert is_subsequence("abc", "qaxbcy") == True
  assert is_subsequence("abc", "axyb") == False
  assert is_subsequence("abc", "abc") == True
  assert is_subsequence("abc", "") == False
  assert is_subsequence("", "qaxbcy") == False
output: def is_subsequence(str1, str2):
      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)