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