Steps to reproduce:
1. Create a new project (any template)
2. Import the attached "USS2_EmptyValue.uss" asset
(or create a .uss file containing: .probe

{ width: ; }

)
3. Observe the Console window

Actual results: An internal exception is reported as the import error, with no mention of the property or of what to fix, and the stylesheet imports with zero rules

Expected results: A validation message naming the property and the expected syntax, in the same style as other invalid values, e.g. "Expected (<length-percentage> | auto) but found ''"

Reproducible with versions: 6000.0.84f1, 6000.3.25f1, 6000.6.2f1, 6000.7.0b2, 7000.0.0a1

Tested on (OS): macOS 27

Assets/USS2_EmptyValue.uss (line 6): error: Internal import error: Index was out of range. Must be non-negative and less than the size of the collection.
Parameter name: index
  at System.Collections.Generic.List`1[T].get_Item (System.Int32 index) [0x00009] in <24665903b31c4531addc9328d72b98a1>:0 
  at ExCSS.TokenValue.get_Item (System.Int32 index) [0x00000] in <913a2fd137ce4c6aad9aeda3f8dd5a38>:0 
  at UnityEditor.UIElements.StyleSheets.StyleSheetImporterImpl.GetPropertyLine (ExCSS.Property property) [0x00000] in /Users/bokken/build/output/unity/unity/Editor/Mono/UIElements/StyleSheets/StyleSheetImporterImpl.cs:1263 
  at UnityEditor.UIElements.StyleSheets.StyleSheetImporterImpl.VisitSheet (ExCSS.Stylesheet styleSheet) [0x00089] in /Users/bokken/build/output/unity/unity/Editor/Mono/UIElements/StyleSheets/StyleSheetImporterImpl.cs:1281 
  at UnityEditor.UIElements.StyleSheets.StyleSheetImporterImpl.ImportParserStyleSheet (UnityEngine.UIElements.StyleSheet asset, ExCSS.Stylesheet styleSheet, System.Collections.Generic.List`1[T] errors) [0x0004c] in /Users/bokken/build/output/unity/unity/Editor/Mono/UIElements/StyleSheets/StyleSheetImporterImpl.cs:1148 
UnityEngine.StackTraceUtility:ExtractStackTrace ()
UnityEditorInternal.InternalEditorUtility:ProjectWindowDragV2 (UnityEditor.HierarchyIterator,bool)
UnityEditor.GUIView:ProcessEvent (UnityEngine.EntityId,intptr,bool&)