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 or substring is empty, return -1.
  assert find_substring("hello", "ll")==2
  assert find_substring("hello", "world")==-1
  assert find_substring("hello", "")==-1
  assert find_substring("", "hello")==-1
  assert find_substring("mississippi", "issip")==4
output: def find_substring(s, sub):
      if not s or not sub:
          return -1
      return s.find(sub)