input: Write a function to print a random element from a given list of strings.
  assert select_random(["apple", "banana", "mango", "pear", "grape"]) in ["apple", "banana", "mango", "pear", "grape"]
  assert select_random([])==None
output: import random
  
  def select_random(lst):
      if lst:
          return random.choice(lst)
      else:
          return None