Scenes preview icon (not thumbnail image) may be wrongly set when the root node has no script

### Tested versions

v4.3.dev.custom_build [7d151c838]

### System information

linux

### Issue description

- may be related to (didn't investigate) https://github.com/godotengine/godot/pull/77932

| the scene (root has no script). The first child has a custom icon | the scene thumbnail (notice the icon) |
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- if a child of a scriptless root has a custom icon, the scene thumbnail will show the child's icon as a type.
- adding an empty script to the root solves the problem.

### Steps to reproduce

- create scene with no script on the root
- add a child node with any custom icon
- save it and look at the file icon

### Minimal reproduction project (MRP)

any

No file, test, or code entry point is named. Begin by reproducing the issue in Godot with a scriptless root and a child with a custom icon, then inspect the scene thumbnail and file-icon generation path. Done means the preview no longer uses the child’s custom icon as the scene type when the root has no script.

Written by the indexing model from the issue text.