input: Write a function to find a substring within a given string and return the starting index of the substring. If the substring is not found, return -1. If the substring is an empty string, return 0.
  assert find_substring("hello world", "world")==6
  assert find_substring("hello world", "hello")==0
  assert find_substring("hello world", "o w")==4
  assert find_substring("hello world", "notfound")==-1
  assert find_substring("hello world", "")==0
output: def find_substring(s, sub):
      if sub == "":
          return 0
      index = s.find(sub)
      return index