TransactGetItems answers {} for an item its projection leaves empty #867

What is wrong

When a Get member of TransactGetItems has a ProjectionExpression that names no attribute of an existing item, Amazon DynamoDB returns the response entry {"Item": {}}. ExtendDB returns {}, the entry for a missing item, so a client cannot tell an existing item from a missing one.

Measured in us-east-1 on 2026-11-05. An existing item with the projection nope, or with a.b where a is a number, gives {"Item": {}}. A missing item gives {}. The projection l[7] on an item whose list l has fewer than 8 elements gives {} on DynamoDB as well.

PartiQL ExecuteTransaction on #411 now projects its reads itself and gives the same answers as DynamoDB, so this is about the classic operation only.

Where

crates/engine/src/transact_get_items.rs, where the projected item becomes the response entry.

What to change

1. Return {"Item": {}} for an existing item whose projection leaves no attribute, and {} for a missing item.
2. Measure the l[7] case again before changing it. Then add tests for the three cases to the TransactGetItems wire tests, against both targets.