NullReferenceException is thrown in BuilderAssetModificationProcessor.IsUxml() when null string is passed as the assetPath parameter

Reproduction steps:
1. Open the attached “IN-121235.zip” project
2. From the top menu click Debug > Show Bug
3. Observe the Console

Expected result: No errors
Actual result: NullReferenceException is thrown

Reproducible with: 2023.1.0b1, 6000.0.60f1, 6000.2.8f1, 6000.3.0b7, 6000.4.0a3

Reproducible on: macOS 26.0.1 (M1 Max), Windows 11
Not reproducible on: No other environment tested

Notes:

• The root cause of this issue is that AssetModificationProcessor.OnWillSaveAssets() can pass arrays containing null entries under certain conditions (asset deletion during save, concurrent operations, etc.). The IsUxml() method does not handle this case
• The intermittent nature of the bug in production suggests that Unity's asset pipeline occasionally passes null entries in the paths array during: 1) Concurrent asset operations, 2) Asset deletion during save operations, 3) Long-running main thread operations that stall asset processing, 4) Batch build scenarios