input: Write a function to find the start index of a substring in a given string. If the substring is not found, return -1. If the given string is empty, return -1.
  assert find_substring("Hello, world!", "world") == 7
  assert find_substring("Hello, world!", "o") == 4
  assert find_substring("Hello, world!", "Python") == -1
  assert find_substring("", "a") == -1
  assert find_substring("Hello, world!", "") == -1
output: def find_substring(s, sub):
      if not s or not sub:
          return -1
      index = s.find(sub)
      return index if index != -1 else -1