input: Write a function to evaluate a given string containing a numerical expression and return the result. If the string is empty, return None.
  assert eval_expr("1+2*3")==7
  assert eval_expr("4/2")==2
  assert eval_expr("9-5")==4
  assert eval_expr("2**3")==8
  assert eval_expr("")==None
output: def eval_expr(s):
      if not s:
          return None
      return eval(s)