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

[koromkorom]

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.
• [X] I have read the guide for submitting bug reports.
• [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:

    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