Reproduction steps:
1. Open the “EmptyNameDuplication“ project
2. Expand the “Treestump.glb“ asset
3. Select and duplicate any Material (or Mesh) that has an empty name

Expected result: a duplicate of the selected sub-asset appears in Assets (e.g. named “Material.mat”)
Actual result: a wrong extension / invalid asset named “_mat” (or “_mesh”) appears in Assets, and an error is logged in Console

Reproduced in: 2020.3.45f1 (without the error), 2021.3.19f1, 2022.2.7f1, 2023.1.0b3, 2023.2.0a2

Reproduced using: macOS 13.0.1 (Intel), Windows 11 (by user)

Full error: “CreateAsset() should not be used to create a file of type '' - consider using AssetDatabase.ImportAsset() to import an existing '' file instead or change the file type to '*.asset'. This error will in a future release be changed to an exception.
UnityEditor.EditorApplication:Internal_CallGlobalEventHandler ()”