batches: allow batch change `last_applied_at` to be nullable

Created by: courier-new

Part 1 of EM2.4 (https://github.com/sourcegraph/sourcegraph/issues/26796), which adds the DB migration to allow last_applied_at on the batch change to be nullable, as it would be for an empty batch change.

For the down migration, I chose to set the column value for empty batch changes to the "zero" timestamp aka the unix epoch time as the closest approximation to not having an applied-at date, but realistically this isn't a change we should anticipate needing to revert for any instance that has empty batch changes active on it.