fix(renderer): clear paths for null children

[0d00ciallo0721]

Fixes #4033.

Dash serializes a callback returning None for children as an explicit null value. The renderer treated all falsy child values as if the callback had omitted the property, so it skipped path recomputation. Removed descendants stayed in the Redux path table, allowing their callbacks to keep firing and preventing a clean remount. The renderer now skips recomputation only when the output property is absent (undefined).

Contributor Checklist

• Scope: recompute paths for explicit children values, including null.
  • Add a browser integration regression for removing and remounting callback-generated children.
• Run the relevant tests locally.
• Add regression coverage for the bug.

Validation

• python -m pytest tests/integration/callbacks/test_children_none.py --headless -xvs — passed; the same test failed on the unmodified base when the removed child remained in the path table.
• python -m pytest tests/integration/callbacks/test_layout_paths_with_callbacks.py tests/integration/callbacks/test_wildcards.py -k 'cblp001 or cbwc003 or cbwc011 or cbwc014' --headless -xvs — 4 passed.
• npm test in dash/dash-renderer — 123 passed.
• Renderer build and npm run private::lint.renderer — successful. ESLint reported three warnings in untouched files.
• Black, Flake8, and git diff --check — passed.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud