Add 0xF2 code for empty DNS response

Closes #1685

requested review from @Diziet

I think this should definitely wait until the discussion here tor!830 (comment 3094607) is complete...

As I write in torspec#278 (closed) I think this spec change ought to be reverted. I'll leave this MR open for now until the dust has settled over there.

Thanks, but: the spec change has been reverted. If I'm not mistaken, Arti doesn't have the bug described in tor#40248 (closed). So I'm closing this MR now.

closed