Is ever answering "No Basis for Judgement" on a reference request a red-flag
Usually you can supply context either as comments on the form or as a separate letter.  This is where you can explain you “no basis for judgement”.  It is not uncommon (although maybe so for this specific question): you may simply not have the historical pool of students to make an informed comparison.
You can use comments or a letter to simply restate what you just included in your post, and this should be enough to justify an honest opinion AND provide useful additional information for a committee.