There is no standard rule, but it's good practice to use "NA" instead of leaving a field blank so that we know that the supplier didn't just overlook the field. I can't think of an instance where the part/drawing revision information wouldn't be applicable.
It's often helpful to compose your own PPAP requirements as a supplement to the AIAG requirements. You can see an example (which I wrote) here (PDF). Note that in the section dealing with the PSW there's a requirement for not leaving spaces blank.
