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.
  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("", "World", "Python")==""
output: def replace_string(input_str, old_str, new_str):
      if not input_str:
          return ""
      return input_str.replace(old_str, new_str)