How to reproduce:
1. Create a new project and install the Visual Effects package
2. Create any VFX Graph and select it
3. In the Inspector, under the "Template" section, check the "Use as Template"
4. Enter anything into the "Name" field and leave the "Thumbnail" field empty
5. In the Project window, open the VFX template window
6. Scroll to the bottom, select the added template and observe the details tab

Expected result: There is a default thumbnail
Actual results: There is no default thumbnail

Reproducible with: 6000.3.0a5
Can't test with: 6000.0.55f1, 6000.1.15f1, 6000.2.0f1 (no such functionality)

Reproducible on: macOS Sequoia 15.5 (M1)
Not reproducible on: No other environment tested