Framework Import Dynamic Attributes

Trigger: A framework resource uses ImportStatePassthroughID and a required schema.DynamicAttribute.

Fix: In Read, treat null, unknown, or zero-value dynamic attributes as absent imported state, fetch backend values, and only overwrite fetched state with prior dynamic state when the prior value is known.

Why: Terraform import initially supplies only the imported ID, so converting a missing dynamic value as if it came from config can fail before the backend read populates state.