You should remove the zero and leave it blank (even if it had a zero on the form, you would still want to leave it blank in TurboTax). If you are having any trouble removing the zero through the interview, try switching to Forms mode (it shows you are using the desktop software) and directly edit box 9a on the Form 1099-R (it will likely have a red exclamation point next to the form and the box will be highlighted pink).