input: Write a function to replace all occurrences of a given string with an alternate string. If the input string is empty then return the input string itself.
  
  def replace_string(input_string, target_string, replacement_string):
      if not input_string:
          return input_string
      return input_string.replace(target_string, replacement_string)
  
  assert replace_string("Hello World!", "World", "Python") == "Hello Python!"
  assert replace_string("Hello World!", "Hello", "Hi") == "Hi World!"
  assert replace_string("Hello World!", "o", "e") == "Helle Werld!"
  assert replace_string("Hello World!", "xyz", "abc") == "Hello World!"
  assert replace_string("", "World", "Python") == ""
output: 
  def replace_string(input_string, target_string, replacement_string):
      if not input_string:
          return input_string
      return input_string.replace(target_string, replacement_string)
  