Upgrading "Do Nothing" model to one with a single action fails to decrypt old items

### Problem:

According to our documentation it should always be possible to add new attributes to our model without issue: https://docs.aws.amazon.com/dynamodb-encryption-client/latest/devguide/data-model.html#add-attribute

However, if you start with data encrypted using
```
actions = AttributeActions(
default_action=CryptoAction.DO_NOTHING
)
```

And update to using
```
actions = AttributeActions(
default_action=CryptoAction.DO_NOTHING, attribute_actions={"someNewField": CryptoAction.ENCRYPT_AND_SIGN}
)
```

You run into issues. This is because data under the first model doesn't have a material description or signature written with it. Once the model is updated to include an action other than `DO_NOTHING`, it always expects there to be a material description and signature, *even if the record it's attempting to decrypt doesn't include `someNewField` yet*.

### Solution:

We should probably update the logic here to also pass through if the item under decrypt specifically doesn't have attributes where encryption or signing is needed, even if the attributeActions includes an encrypt or sign action for a non-present field.

https://github.com/aws/aws-dynamodb-encryption-python/blob/25c7c3d80bfbe0deb661b4beb86f61b8b2f8545e/src/dynamodb_encryption_sdk/encrypted/item.py#L176-L178

https://github.com/aws/aws-dynamodb-encryption-python/blob/25c7c3d80bfbe0deb661b4beb86f61b8b2f8545e/src/dynamodb_encryption_sdk/structures.py#L137-L148

Beginne in src/dynamodb_encryption_sdk/encrypted/item.py ungefähr bei den Zeilen 176–178 und in src/dynamodb_encryption_sdk/structures.py ungefähr bei den Zeilen 137–148. Stelle den Übergang von einem ausschließlich DO_NOTHING verwendenden AttributeActions-Modell zu einem Modell nach, das für someNewField ENCRYPT_AND_SIGN hinzufügt, und verfolge anschließend die Entschlüsselung eines älteren Elements ohne dieses Feld. Erledigt ist die Aufgabe, wenn alte Elemente ohne Attribute, die eine Verschlüsselung oder Signierung erfordern, weiterhin unter dem aktualisierten Modell entschlüsselt werden können.

Vom Indexierungsmodell aus dem Issue-Text verfasst.