Add fallback default value for private fields in __setstate__ of BaseModel

[anhpham1509]

Change Summary

• Instead of dictionary access, it's safe to use .get for private model attributes with empty dictionary fallback value.
• This allows compatibility to v1 BaseModel when using pickle with reference to data class as well.

Related issue number

Checklist

• The pull request title is a good summary of the changes - it will be used in the changelog
• Unit tests for the changes exist
• Tests pass on CI
• Documentation reflects the changes where applicable
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

Selected Reviewer: @adriangb

[codspeed-hq]

CodSpeed Performance Report

Merging #9584 will not alter performance

_{Comparing anhpham1509:fix/allow-backward-compatibility-with-v1-basemodel (b640268) with main (93d6015)}

Summary

✅ 13 untouched benchmarks

[anhpham1509]

please review

[sydney-runkle]

This looks good to me, thank you for the fix!