input: Write a function to replace a given word in a string with another word provided. The function should return the modified string.
  assert replace_word("I love walking on the beach", "walking", "swimming")=="I love swimming on the beach"
  assert replace_word("I love walking on the beach", "the beach", "the pool")=="I love walking on the pool"
  assert replace_word("I love walking on the beach", "I", "We")=="We love walking on the beach"
  assert replace_word("I love walking on the beach", "", "something")=="I love walking on the beach"
  assert replace_word("", "walking", "swimming")==""
output: def replace_word(s, old_word, new_word):
      if old_word == "":
          return s
      return s.replace(old_word, new_word)