ProSeries has always railroaded you into filling out the 3903 any time there's a code P.  Maybe it hasn't been a critical efile stopping error (I don't know) but it's definitely a "red box" error if you don't link the W-2 code P to a 3903, "Form W-2 Worksheet (EmployerName): Link for box 12 code P to Form 3903 must be established."

I've always thought of it (perhaps incorrectly) as something akin to filing a 2441 to justify a W-2 box 10 amount is not taxable.  I looked at the 3903 from 2017 and there's no "who must file" section but I suspect you're right, there's no IRS requirement to file if you have no unreimbursed expenses.

Rick