fix: handle cleared optional file tool inputs

[stevenup]

Summary

• Reset cleared optional file and files form inputs to an unset null constant in the workflow editor.
• Ignore legacy empty optional variable selectors at runtime while preserving validation for required and malformed non-empty selectors.
• Add frontend and backend regression coverage for empty-string and empty-list selector states.

Fixes #39045

Screenshots

Not applicable. This is a behavioral fix with no visual UI changes.

Test plan

• uv run --project api pytest api/tests/unit_tests/core/tools/test_tool_manager.py -q (50 passed)
• make lint && make type-check
• cd web && pnpm test app/components/workflow/nodes/_base/components/__tests__/form-input-item.branches.spec.tsx app/components/workflow/nodes/_base/components/__tests__/form-input-item.spec.tsx (17 passed)
• cd web && pnpm exec vp staged --cwd .. --diff upstream/main...HEAD --fail-on-changes
• git diff --check upstream/main...HEAD

Checklist

• This change does not require a documentation update.
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• Documentation is unchanged because this fix does not alter user-facing configuration or APIs.
• I ran make lint && make type-check (backend) and the frontend staged checks for the full PR diff.