input: Write a function that takes a sentence as input and returns a list of verbs extracted from the sentence using the Natural Language Toolkit (NLTK). If there are no verbs, return an empty list. If the input sentence is empty, return an empty list as well.
  
  assert extract_verbs("The cat jumped over the moon.")==['jumped']
  assert extract_verbs("She is reading a book.")==['is', 'reading']
  assert extract_verbs("This is a test.")==['is']
  assert extract_verbs("")==[]
  assert extract_verbs("No verbs here.")==[]
output: def extract_verbs(sentence):
      if not sentence:
          return []
      words = nltk.word_tokenize(sentence)
      pos_tags = nltk.pos_tag(words)
      verbs = [word for word, pos in pos_tags if pos.startswith('V')]
      return verbs