v1.2.1 for Minecraft 1.12.2, Forge 14.23.5.2768

Fixes a minor issue with low-power enchants introduced in v1.2.

When there are sufficiently few bookshelves, the power level can be low enough that no enchant is selected for a slot. The mod had a regression where this wasn't handled properly.

Also, added update.json tracking to the mod, so future releases will be notified in-game.