fix(flutter/datastore): Nullable custom type arrays

[dnys1]

• PR title and description conform to Pull Request guidelines.

Issue #, if available:

aws-amplify/amplify-flutter#3575

Description of changes:

When a custom type array is not required and null is returned from AppSync, deserialization will fail since getAsJsonArray will throw for null values.

This resolves the issue by checking if a field is null before attempting deserialization.

How did you test these changes?

Added test case for null custom type array.

Documentation update required?

• No
• Yes (Please include a PR link for the documentation update)

General Checklist

• Added Unit Tests
• Added Integration Tests
• Security oriented best practices and standards are followed (e.g. using input sanitization, principle of least privilege, etc)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

Merging #2565 (14f500b) into main (07c0de3) will increase coverage by 0.01%.
The diff coverage is 100.00%.

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+ Coverage   41.43%   41.44%   +0.01%     
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  Files         874      874              
  Lines       27644    27648       +4     
  Branches     3883     3884       +1     
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+ Hits        11455    11460       +5     
- Misses      14960    14961       +1     
+ Partials     1229     1227       -2