input: Write a function to find the start index of a substring in a given string. If the substring is not found, return -1.
  assert find_substring("hello world", "world") == 6
  assert find_substring("hello world", "lo") == 3
  assert find_substring("hello world", "cat") == -1
  assert find_substring("hello world", "hello") == 0
  assert find_substring("hello world", "") == -1
output: def find_substring(s, sub):
      if not sub:
          return -1
      index = s.find(sub)
      return index if index != -1 else -1