Error if no masks found

[nonnull-ca]

If there are no masks found, this errors out here, due to said reduce having no default value.

I can reproduce this fairly easily:

1. Leave settings on defaults aside from the below.
2. Input this image.
3. Enable Segment Anything & CLIP.
4. Set SAM model to sam_vit_h_4b8939.pth.
5. Set predicted_iou_threshold / stability_score_threshold / clip_threshold to 1.
6. Set a segmentation prompt of 'asdf'.
7. Enable CascadePSP
8. Set CascadePSP to fast.
9. Generate.

(Most of these steps are probably unnecessary. Ditto, I realize that setting a threshold to 1 is somewhat silly.)

Said error is also fairly ungraceful in the UI: a red 'Error' and that's it.

Maybe either a) pass a default all-ones mask or b) if no masks pass the threshold then pick one mask with the highest confidence?

[mattyamonaca]

Thanks!
I'll try to modify it in the direction of option B.