Users are prompted 2 times if they want to discard the Shader Graph when closing Shader Graph Window when the Asset has already been deleted

Steps to reproduce:

1. Create a new URP 3D Project
2. In the Project Browser create a new Empty Shader Graph Asset and open it in Shader Graph
3. In the Project Browser delete the Empty Shader Graph Asset
4. When at first prompted choose "Cancel" and observe the Label of the Shader Graph Window
5. Right click the left over Shader Graph Window's label and choose "Close Tab" and cancel again when prompted
6. Observe the Window's label as it's now marked dirty
7. Right click the left over Shader Graph Window's label and choose "Close Tab" again
8. Observe that now the first prompt also opens as "Unsaved Changes Detected" is now shown

Actual results: Cancelling the closing of the left over Shader Graph Window will mark it dirty, and will prompt users with the "Unsaved Changes Detected" prompt, making users go through 2 discarding prompts to close the Window

Expected results: Cancelling the closing of the left over Shader Graph Window does not mark the Asset Dirty, and when closing the window subsequently users are only prompted once, not twice

Reproducible with versions: 6000.0.76f1, 6000.3.16f1, 6000.4.10f1, 6000.5.0b9, 6000.6.0a6

Tested on (OS): Windows 11