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 original string.
  assert replace_string("Hello World!", "World", "Python")=="Hello Python!"
  assert replace_string("The quick brown fox", "the", "a")=="The quick brown fox"
  assert replace_string("The quick brown fox", "The", "A")=="A quick brown fox"
  assert replace_string("", "any", "anything")==""
  assert replace_string("aaa", "a", "b")=="bbb"
output: def replace_string(original, target, replacement):
      if target == "":
          return original
      return original.replace(target, replacement)