input: Write a function to replace a given string in a sentence with another string. If the given string is not found in the sentence, return the original sentence.
  assert replace_word("I love walking on the beach", "walking", "swimming")=="I love swimming on the beach"
  assert replace_word("I am reading a book", "reading", "writing")=="I am writing a book"
  assert replace_word("She plays the piano", "violin", "guitar")=="She plays the piano"
  assert replace_word("", "anything", "something")==""
  assert replace_word("This is a test", "", "new")=="This is a new test"
output: def replace_word(sentence, old_word, new_word):
      if old_word == "":
          return sentence.replace(" ", new_word + " ")
      return sentence.replace(old_word, new_word)