input: Write a function to replace all occurrences of a given string with an alternate string. If the given string is empty then return the input string.
  assert replace_string("Hello World!", "World", "Python")=="Hello Python!"
  assert replace_string("This is a test", "is", "X")=="ThX X a test"
  assert replace_string("", "anything", "else")==""
  assert replace_string("No change", "not found", "replacement")=="No change"
  def replace_string(input_string, target_string, replacement_string):
      return input_string.replace(target_string, replacement_string)
output: def replace_string(input_string, target_string, replacement_string):
      if target_string == "":
          return input_string
      return input_string.replace(target_string, replacement_string)