input: Write a function to extract the verbs from a given sentence using the NLTK library. If the sentence is empty, return an empty list. The function should take a string as input and return a list of verbs.
  
  assert extract_verbs("The cat jumped over the moon.")==['jumped']
  assert extract_vars("She sings beautifully.")==['sings']
  assert extract_vars("Run to the store.")==['Run', 'store']
  assert extract_vars("")==[]
  assert extract_vars("The quick brown fox")==[]
output: def extract_verbs(sentence):
      if not sentence:
          return []
      from nltk import pos_tag, word_tokenize
      words = word_tokenize(sentence)
      tagged = pos_tag(words)
      verbs = [word for word, pos in tagged if pos.startswith('VB')]
      return verbs