input: Write a function to replace all occurrences of a given string with an alternate string in the input 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("The quick brown fox jumps over the lazy dog", "The", "A") == "A quick brown fox jumps over A lazy dog"
  assert replace_string("", "any", "replacement") == ""
  assert replace_string("Repeat Repeat Repeat", "Repeat", "Go") == "Go Go Go"
output: def replace_string(input_str, old_str, new_str):
      if not input_str:
          return ""
      return input_str.replace(old_str, new_str)