input: Write a function named `replace_word` that takes three parameters: `sentence`, `old_word`, and `new_word`. The function should return a new sentence where all occurrences of `old_word` in `sentence` are replaced with `new_word`. If `sentence` is empty, the function should return an empty string.  
  assert replace_word("I love walking on the beach", "walking", "swimming")=="I love swimming on the beach"  
  assert replace_word("He is always running", "running", "jogging")=="He is always jogging"  
  assert replace_word("", "walking", "swimming")==""  
  assert replace_word("programming is fun", "fun", "exciting")=="programming is exciting"
output: def replace_word(sentence, old_word, new_word):
      if not sentence:
          return ""
      return sentence.replace(old_word, new_word)