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 world")=="hll wrld"
  assert filter_vowels("python programming")=="pythn prgrmmng"
  assert filter_vowels("aeiou")==""
  assert filter_vowels("bcdfg")=="bcdfg"
  assert filter_vowels("")==""
output: def filter_vowels(s):
      vowels = "aeiouAEIOU"
      return ''.join([char for char in s if char not in vowels])