input: Write a function to determine if a given string str1 is a subsequence of another string str2. A subsequence is a sequence that can be derived from another sequence by deleting some or no elements without changing the order of the remaining elements. Return True if str1 is a subsequence of str2, otherwise return False.
  assert is_subsequence('abc', 'qaxbcy') == True
  assert is_subsequence('axc', 'qaxbcy') == False
  assert is_subsequence('abc', 'abc') == True
  assert is_subsequence('abc', '') == False
  assert is_subsequence('', 'qaxbcy') == True
output: def is_subsequence(str1, str2):
      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)