fix(workflows): reject malformed step catalog roots on add

[marcelsafin]

Description

StepCatalog.add_catalog() treated any falsy parsed YAML root as an empty mapping. A config containing [], 0, false, or an empty string was therefore overwritten when adding a catalog instead of being reported as corrupt.

This distinguishes empty/null documents from present non-mapping roots and preserves malformed files unchanged.

Testing

• Tested locally with uv run specify --help
• Ran existing tests with uv sync && uv run pytest (6,655 passed, 177 skipped)
• Added parameterized regressions for falsy non-mapping config roots
• uvx ruff@0.15.0 check src tests clean

AI Disclosure

• I did not use AI assistance for this contribution
• I did use AI assistance (describe below)

Implemented autonomously by GitHub Copilot CLI (model: GPT-5.6 Sol) under human direction; failing regressions were added before the fix, then the full suite and lint were run locally. Commit includes Assisted-by and Co-authored-by trailers.

[Copilot]

Pull request overview

Fixes StepCatalog.add_catalog() to reject malformed falsy YAML roots without overwriting them.

Changes:

• Treats only empty/null YAML documents as empty mappings.
• Adds regression coverage for falsy non-mapping roots and file preservation.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/specify_cli/workflows/catalog.py	Validates parsed YAML root types before writing.
tests/test_workflows.py	Tests rejection and preservation of malformed configs.

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