input: Write a function to evaluate a given string containing a numerical expression and return the result. If the string is empty, return 0.
  assert eval_expr("2+3*4")==14
  assert eval_expr("5**2")==25
  assert eval_expr("10/2")==5.0
  assert eval_expr("9-5")==4
  assert eval_expr("")==0
output: def eval_expr(s):
      if not s:
          return 0
      return eval(s)