input: Write a function to find the start index of a given substring in a provided string. If the substring is not found, the function should return -1. If the provided string is empty, the function should return -1.
  assert find_substring("hello world", "world")==6
  assert find_substring("hello world", "hi")==-1
  assert find_substring("programming", "gram")==3
  assert find_substring("programming", "pro")==0
  assert find_substring("", "test")==-1
output: def find_substring(s, sub):
      if not s:
          return -1
      index = s.find(sub)
      return index