input: Write a function to evaluate a numerical expression given as a string and return the result. If the input string is empty, return 0.
  
  assert eval_expr("2+3*4")==14
  assert eval_expr("(2+3)*4")==20
  assert eval_expr("10/2")==5.0
  assert eval_expr("10//2")==5
  assert eval_expr("")==0
output: def eval_expr(s):
      if not s:
          return 0
      return eval(s)