input: Write a function to extract all verbs from a given sentence using nltk. The function should take a sentence as input and return a list of verbs. If the sentence is empty, the function should return an empty list.
  import nltk
  
  def extract_verbs(sentence):
      if not sentence:
          return []
      tokens = nltk.word_tokenize(sentence)
      tags = nltk.pos_tag(tokens)
      verbs = [word for word, pos in tags if pos.startswith('V')]
      return verbs
  
  assert extract_verbs("The cat jumped over the moon.")==['jumped']
  assert extract_verbs("She runs quickly every morning.")==['runs', 'quickly']
  assert extract_verbs("I am happy.")==['am']
  assert extract_verbs("")==[]
output: 
  def extract_verbs(sentence):
      if not sentence:
          return []
      tokens = nltk.word_tokenize(sentence)
      tags = nltk.pos_tag(tokens)
      verbs = [word for word, pos in tags if pos.startswith('V')]
      return verbs
  