input: Write a function to filter out all the vowels from a given string. If the string is empty, return an empty string.
  assert filter_vowels('hello')=='hll'
  assert filter_vowels('python')=='pythn'
  assert filter_vowels('aeiou')==''
  assert filter_vowels('')==''
output: def filter_vowels(s):
      vowels = "aeiouAEIOU"
      return ''.join([char for char in s if char not in vowels])