It's actually not possible to indicate that any amount is exempt from the 10% early-distribution penalty because only a taxable amount is subject to such a penalty. Since the taxable amount shown in box 2a is zero, there is already no penalty to eliminate.
As to why code J is present in box 7, the code J is necessary to identify that the return of contribution is from a Roth IRA and not from some other kind of retirement account. It's just a quirk of how the IRS requires the codes to be used and, when combined with code P or code 8, does not mean that the distribution is subject to an early-distribution penalty
