input: Write a function to find a substring in a given string and return the start index of it. If the substring is not present, return -1. If the given string is empty, return -1.
  assert find_substring("Hello World", "World") == 6
  assert find_substring("Hello World", "Python") == -1
  assert find_substring("Hello World", " ") == 5
  assert find_substring("Hello World", "") == -1
  assert find_substring("", "Hello") == -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