input: Write a function to replace a given string with another string in a sentence. If the old word 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 love walking on the beach", "beach", "ocean")=="I love walking on the ocean"
  assert replace_word("I love walking on the beach", "running", "jogging")=="I love walking on the beach"
  assert replace_word("", "walking", "swimming")==""
  assert replace_word("I love walking on the beach", "", "jumping")=="I love walking on the beach"
output: def replace_word(sentence, old_word, new_word):
      if old_word in sentence:
          return sentence.replace(old_word, new_word)
      return sentence