Node-RED: use msg.topic when Tag field is empty

[smort82]

📌 Description

This PR updates the Node-RED get-tag and set-tag nodes to allow using msg.topic
as the tag reference when the Tag field in the UI is left empty.

Behavior:

• If Tag field is set → use it
• If Tag field is empty → fallback to msg.topic
• Error only when both are missing

Backwards compatible: existing flows with Tag set are unchanged.

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🧪 Type of Change

Please mark the relevant option:

• Bug fix
• [ x] New feature
• Refactoring
• Documentation
• Other

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🚫 Build Artifacts Check

Please confirm:

• [x ] I did NOT commit /client/dist
• [x ] I did NOT commit generated Angular build output
• [ x] Only source files are included

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🔍 Checklist

• [x ] Code follows project coding standards
• [x ] I tested my changes locally
• [ x] Documentation updated if required
• [x ] Issue opened (for major changes)

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📚 Documentation Checklist (if applicable)

• The documentation builds locally with mkdocs serve
• All links were tested
• Images use relative paths (e.g. images/example.png)
• No references to the old GitHub Wiki
• Navigation updated in mkdocs.yml (if required)

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📝 Additional Notes

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