unnecessary_non_null_assertion for `updatedAt` and `createdAt` in `toString()` from dart generator

### Before opening, please confirm:

- [X] I have installed the latest version of the Amplify CLI (see above), and confirmed that the issue still persists.
- [X] I have [searched for duplicate or closed issues](https://github.com/aws-amplify/amplify-codegen/issues?q=is%3Aissue+).
- [X] I have read the guide for [submitting bug reports](https://github.com/aws-amplify/amplify-codegen/blob/main/CONTRIBUTING.md#bugs).
- [X] I have done my best to include a minimal, self-contained set of instructions for consistently reproducing the issue.

### How did you install the Amplify CLI?

curl

### If applicable, what version of Node.js are you using?

v18.20.2

### Amplify CLI Version

12.12.0

### What operating system are you using?

Mac

### Amplify Codegen Command

codegen models

### Describe the bug

Generation of default DateTime fields for flutter creates `unnecessary_non_null_assertion` warning in `toString()` method for the exclamation marks:

```dart
buffer.write("createdAt=" + (_createdAt != null ? _createdAt!.format() : "null") + ", ");
buffer.write("updatedAt=" + (_updatedAt != null ? _updatedAt!.format() : "null"));
```

### Expected behavior

No such warnings.

### Reproduction steps

1. Create any schema field in Amplify Studio
2. `amplify pull` in flutter project
3. `amplify codegen models`

### GraphQL schema(s)

```graphql
# Put schemas below this line
type Message @model @auth(rules: [{allow: public}]) {
id: ID!
text: String
}

```

### Log output

```
# Put your logs below this line
The '!' will have no effect because the receiver can't be null.
Try removing the '!' operator.

```

### Additional information

_No response_