input: Write a function to replace a given string with another string in a sentence. The function should return the modified sentence. If the old word is not found in the sentence, the function should return the original sentence unchanged.
  
  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", "beach", "mountain")=="I love walking on the mountain"
  assert replace_word("I love walking on the beach", "surfing", "swimming")=="I love walking on the beach"
  assert replace_word("", "walking", "swimming")==""
  assert replace_word("I love walking on the beach", "", "running")=="I love walking on the beach"
output: def replace_word(sentence, old_word, new_word):
      if old_word == "":
          return sentence
      return sentence.replace(old_word, new_word)