input: Write a python function to generate a random string of specified length containing only vowels.
  assert len(get_random_string(5)) == 5
  assert all(char in 'aeiou' for char in get_random_string(10))
  assert len(get_random_string(0)) == 0
output: import random
  
  def get_random_string(length):
      vowels = 'aeiou'
      return ''.join(random.choice(vowels) for _ in range(length))