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.
  assert extract_verbs("The cat jumped over the moon.")==['jumped']
  assert extract_verbs("She was running fast.")==['was', 'running']
  assert extract_verbs("I love programming.")==['love', 'programming']
  assert extract_verbs("")==[]
output: import nltk
  from nltk.tokenize import word_tokenize
  from nltk.tag import pos_tag
  
  def extract_verbs(sentence):
      if not sentence:
          return []
      words = word_tokenize(sentence)
      tagged_words = pos_tag(words)
      verbs = [word for word, pos in tagged_words if pos.startswith('VB')]
      return verbs