input: Write a function to replace all occurrences of a given string with an alternate string in the input string. If the given string is empty then return the input string unchanged.
  assert replace_string("Hello World!", "World", "Python")=="Hello Python!"
  assert replace_string("Hello World!", "o", "a")=="Hella Warld!"
  assert replace_string("Hello World!", "Hello", "")==" World!"
  assert replace_string("Hello World!", "", "Python")=="Hello World!"
  assert replace_string("", "", "Python")==""
output: def replace_string(input_str, old_str, new_str):
      if old_str == "":
          return input_str
      return input_str.replace(old_str, new_str)