Fix empty multipart body form fields

[cyphercodes]

Summary

• Avoid indexing multipart body form data when a submitted string field is empty.
• Preserve the existing quote-wrapping behavior while allowing empty strings to parse as JSON string bodies.
• Add regression coverage for a multipart request with a Body string field and file upload.

Tests

• pytest -q tests/test_forms_and_files.py tests/test_params_models.py
• ruff format --check ninja tests
• ruff check ninja tests
• git diff --check

Fixes #1703