Fix #9750: Respect blank=True on model CharField in serializer

[MuraveyApp]

Summary

• Fixes 3.15 raising required error on model CharField with blank=True
• Serializer now respects the model field blank=True setting

Test plan

• Verify CharField(blank=True) does not raise 'This field is required'
• Existing test suite passes

Generated by OwlMind

[browniebroke]

Please direct your AI agent a bit more before sending this for code review. This will at least need a few tests