input: Write a python function to get the first element of a list if the list is not empty; otherwise, return None.
  assert get_first_element([1, 2, 3, 4, 5]) == 1
  assert get_first_element([]) == None
  assert get_first_element([7, 8, 9]) == 7
output: def get_first_element(lst):
      return lst[0] if lst else None