The crafter emits a Redstone signal with locked slots when using a comparator.

Using a Redstone Comparator with the crafter, it emits a Redstone signal when its slots are locked, even if there are no items inside the crafter. I believe the comparator works the same way as if there were items inside the crafter because the Redstone signal changes according to the number of locked slots.

To reproduce this, simply place a crafter and a comparator attached to it, with the signal output facing the opposite direction (images attached).