"Invalid image specified" warning is thrown when leaving an empty field in Cursor in in-scene UI Toolkit

Steps to reproduce:

1. Create and open a new project
2. Create a  UI Builder
3. Assets/Create/UI Toolkit/Standard Elements/Button
4. Select the Button element and scroll down to "Cursor" in the Inspector
5. Assign any valid texture into the Cursor Image field
6. Open the Object Picker and select 'None' to remove the reference
7. Observe the Console

Actual results: "Invalid image specified" warnings are thrown

Expected results: Nothing is thrown

Reproducible with versions: 6000.6.0b1, 6000.6.0b8, 6000.7.0a4

Can’t test with versions: 6000.0.81f1, 6000.3.22f1, 6000.5.8f1, 6000.6.0a1 (Doesn't support editing outside of UI Builder)

Tested on (OS): Windows 11 Pro (25H2)

Notes:

• 4 "Invalid image specified" warnings are thrown{}
• When saving the file, "Invalid URI location" warnings and "USS in 'X' attribute is invalid" are thrown 
• Only happens in in-scene UI Toolkit
• Unsetting the field resolves the warnings until setting it to 'None' again