input: Write a function to replace a given string with another string. The function should return the new sentence after the replacement. The function should take three parameters: the original sentence, the old word, and the new word. If the old word is not found in the sentence, the function should return the original sentence unchanged.
  assert replace_string("I love walking on the beach", "walking", "swimming")=="I love swimming on the beach"
  assert replace_string("Sunny weather is perfect", "weather", "days")=="Sunny days is perfect"
  assert replace_string("Old friends are best", "old", "New")=="Old friends are best"
  assert replace_string("A quick brown fox jumps over the lazy dog", "cat", "dog")=="A quick brown fox jumps over the lazy dog"
  assert replace_string("", "placeholder", "content")==""
output: def replace_string(sentence, old_word, new_word):
      if old_word in sentence:
          return sentence.replace(old_word, new_word)
      return sentence