input: Write a function that takes a string containing a numerical expression and returns the result. The function should evaluate the expression using Python's built-in eval() function. If the input string is empty, the function should return None.
  
  assert eval_expr("2+3*4")==14
  assert eval_expr("10/2")==5.0
  assert eval_expr("5**2")==25
  assert eval_expr("3-6")==-3
  assert eval_expr("0.5+0.5")==1.0
  assert eval_expr("")==None
output: def eval_expr(s):
      if not s:
          return None
      return eval(s)