input: Write a function to evaluate a string containing a numerical expression and return the result. If the input string is empty, return None.
  def eval_expr(expr):
      if not expr:
          return None
      return eval(expr)
  assert eval_expr("2+3")==5
  assert eval_expr("10-5")==5
  assert eval_expr("3*4")==12
  assert eval_expr("16/2")==8
  assert eval_expr("")==None
output: def eval_expr(expr):
      if not expr:
          return None
      return eval(expr)