input: Write a function to replace all occurrences of a given string with an alternate string. If the given string is empty, return the empty string.
  assert replace_string("Hello World!", "World", "Python")=="Hello Python!"
  assert replace_string("Hello World!", "World", "Universe")=="Hello Universe!"
  assert replace_string("Python is awesome", "Python", "Java")=="Java is awesome"
  assert replace_string("Python is awesome", "awesome", "great")=="Python is great"
  assert replace_string("", "any", "none")==""
output: def replace_string(s, old, new):
      if old == "":
          return ""
      return s.replace(old, new)