fix: resolve empty settings validation error in workflow updates (#431)

Fixed critical bug where n8n_update_partial_workflow failed with "request/body
must NOT have additional properties" error when workflows had no settings or
only non-whitelisted settings.

Root Cause:
- cleanWorkflowForUpdate() was sending empty settings: {} objects
- n8n API rejects empty settings as additional properties violation
- Occurred when workflow had no settings or only filtered properties

Changes:
- Modified cleanWorkflowForUpdate() to delete settings property when empty
- Enhanced applyUpdateSettings() to prevent creating empty settings
- Fixed 3 incorrect tests expecting empty settings objects
- Added 2 comprehensive tests for edge cases

Testing:
- All 75 unit tests in n8n-validation.test.ts passing
- Build and type checking successful
- New tests cover all empty settings scenarios

Related: #431, #248
Related n8n issue: n8n-io/n8n#19587

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Conceived by Romuald Członkowski - https://www.aiadvisors.pl/en

package.json

@@ -1,6 +1,6 @@
 {
   "name": "n8n-mcp",
-  "version": "2.22.20",
+  "version": "2.22.21",
   "description": "Integration between n8n workflow automation and Model Context Protocol (MCP)",
   "main": "dist/index.js",
   "types": "dist/index.d.ts",