A refresh upgrade has no prior value, so empty collections read as []

Trigger: the upgraded state holds [] or {} where the plan (configuration without the attribute) holds null, and the first plan after the upgrade is not empty. A read with a prior value does not show it, because keepPriorEmpty keeps the null of the prior state.

Fix: state the null on the field: readEmptyAs: "null" for a list, a set, or a list of objects. A plain string map has no such key, so set it null in AfterFlatten when the attribute rejects an empty value (sizeAtLeast: 1), so the empty map cannot be configured. Test it: upgrade a raw state of every prior version through the protocol harness (UpgradeFrom), compare it with a read of the same alert, and plan the configuration on it (internal/provider/alert_state_upgrade_test.go).

Why: the upgrade passes no prior state to the flatten step, and the API answers an empty collection for a field that the user never set.