input: Write a function named `extract_words` that takes a sentence as input and returns a list of words that contain no vowels (both uppercase and lowercase). If the input sentence is empty, the function should return an empty list.
  
  assert extract_words("This machine learning project is challenging.")==['This', 'machine', 'learning', 'project', 'is', 'challenging.']
  assert extract_words("Python is great!")==['Python', 's', 'grt!']
  assert extract_words("")==[]
output: def extract_words(sentence):
      vowels = "aeiouAEIOU"
      if not sentence:
          return []
      words = sentence.split()
      result = []
      for word in words:
          if all(char not in vowels for char in word):
              result.append(word)
      return result