Fix empty assignment-index backend handling

[FlorianPfaff]

Summary

• Treat empty assignment index collections as no-op updates in the public backend facade.
• Keep empty tuple assignment semantics unchanged.
• Add a backend-contract regression test covering assignment and assignment_by_sum with empty list indices.

Testing

• Not run locally: container GitHub DNS resolution failed, so validation is deferred to repository CI.

[github-actions]

✅⚠️MegaLinter analysis: Success with warnings

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	41.9s
✅ JSON	prettier	7	0	0	0	0.92s
✅ JSON	v8r	7		0	0	5.77s
⚠️ MARKDOWN	markdownlint	66	0	2	0	1.98s
✅ MARKDOWN	markdown-table-formatter	66	1	0	0	0.79s
✅ PYTHON	black	758	3	0	0	35.3s
✅ PYTHON	isort	758	3	0	0	2.33s
✅ REPOSITORY	checkov	yes		no	no	43.09s
✅ REPOSITORY	gitleaks	yes		no	no	16.36s
✅ REPOSITORY	git_diff	yes		no	no	0.11s
✅ REPOSITORY	secretlint	yes		no	no	23.32s
✅ REPOSITORY	syft	yes		no	no	4.85s
✅ REPOSITORY	trivy-sbom	yes		no	no	4.4s
✅ REPOSITORY	trufflehog	yes		no	no	21.84s
✅ YAML	prettier	10	0	0	0	0.7s
✅ YAML	v8r	10		0	0	11.2s
✅ YAML	yamllint	10		0	0	0.56s

Detailed Issues

⚠️ MARKDOWN / markdownlint - 2 errors

.github/pull_request_template.md:1 error MD041/first-line-heading/first-line-h1 First line in a file should be a top-level heading [Context: "## Summary"]
docs/backend-support.md:11:174 error MD060/table-column-style Table column style [Table pipe does not align with header for style "aligned"]

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