Warnings about invalid URIs are thrown when UI Builder's Variable Asset Reference field is set to null

Steps to reproduce:

1. Create and open a new project
2. Create and open a new UXML file in UI Builder
3. Create a new class in the StyleSheets section (create a new USS) and select it
4. Expand the 'Variable' section in UI Builder's Inspector
5. Add a new variable of type 'Asset Reference'
6. Save the USS file
7. Observe the Console

Actual results: " Path/To/USS.uss (line X): warning: Invalid URI location: '''' " warning is thrown

Expected results: Nothing is thrown, and the value for the variable in .uss is either "inline" or "none"

Reproducible with versions: 6000.3.0a1, 6000.3.19f1, 6000.5.2f1, 6000.6.0b2, 6000.7.0a1

Not reproducible with versions: 6000.2.0b2

Can’t test with versions: 6000.0.78f1 (No Variable section)

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

Notes:

• Similarly, this reproduces with the Material section, however, it has a different regression point