fix(blank/unique): add `default` handling for `blank=True` field

[sshishov]

Description

This MR should fix the issue with blank=True field in UniqueTogether constraint. It should not become required all of a sudden.

Fixes https://github.com/encode/django-rest-framework/issues/9750

[sshishov]

I have added tests similar to the tests added in https://github.com/encode/django-rest-framework/pull/9531

[mahdighadiriii]

Hi @sshishov @auvipy @deepakangadi, I'm excited to contribute to DRF as my first contribution! I noticed this PR (#9751) addresses a critical issue (#9750) with blank=True fields in UniqueConstraints, which aligns with my experience in Django/DRF serializers and API optimization.

I can help unblock this by:

• Fixing the typo in test_blank_uqnique_constraint_fields_are_not_required (uqnique → unique).
• Splitting assertions in the test for clarity, as suggested by @deepakangadi.
• Adding char length limit (e.g., max_length=255) to the constraint.
• Resolving conflicts by rebasing on main.
• Checking for overlap with PR #9725, as mentioned by @auvipy.

If this sounds good, I can start working on these changes. Should I update this branch (if permitted) or create a new PR? Thanks for your guidance!

[stale[bot]]

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