fix(backend): fix null margin mappings crashing affinity endpoint

[martingaldeca]

Type of Change

• 🚀 New feature (non-breaking change which adds functionality)
• 🐛 Bug fix (non-breaking change which fixes an issue)
• 🧹 Chore (technical task, refactoring, maintenance)

Description

Briefly describe the changes made in this Pull Request. Explain the solution and the reasoning.
This PR fixes Internal Server Errors occurring on the /api/answers/completed/.../affinity/ endpoints.
The errors were caused by explicitly null JSON values for margin inputs crashing Pydantic validations during compatibility calculations. The changes gracefully fallback to 0 when encountering these cases during _map_axes.

Related Issue

Closes #

How Has This Been Tested?

Please describe the tests that you ran to verify your changes.

• Unit Tests
• Manual Testing

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Screenshots (if applicable)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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