/place jigsaw empty throws an unhandled error

The bug

Structure template pool minecraft:empty (or custom structure template pool with no elements) is valid in /place jigsaw, but returns an unhandled exception.

How to reproduce

1. run

    /place jigsaw empty a 1

2. returns an unexpected error

Expected behavior

Return successful message for template pools that have no element, or the generic jigsaw fail message "Failed to generate jigsaw".