fix(editor): Show placeholder nodes when no trigger nodes available on new canvas (no-changelog)

[alexgrozav]

Summary

https://github.com/user-attachments/assets/8b161ef7-63f8-45f3-b877-0747c971204b

Related Linear tickets, Github issues, and Community forum posts

https://linear.app/n8n/issue/N8N-7780/add-a-trigger-placeholder-disappears-when-pasting-a-non-trigger-node

Review / Merge checklist

• [x] PR title and summary are descriptive. (conventions)
• [ ] Docs updated or follow-up ticket created.
• [x] Tests included.
• [ ] PR Labeled with release/backport (if the PR is an urgent fix that needs to be backported)