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("Hello World!", "o", "a")=="Hella Warld!"
  assert replace_string("Hello World!", "XYZ", "ABC")=="Hello World!"
  assert replace_string("", "World", "Python")==""
  assert replace_string("Hello World!", "", "Python")=="Hello World!"
output: def replace_string(original, target, replacement):
      if target == "":
          return original
      return original.replace(target, replacement)