NuGet.RestoreEx.targets "Object reference not set" during tvos RuntimeTests restore

## Build Information
Build: https://dev.azure.com/dnceng-public/cbb18261-c48f-4abb-8651-8cdcb5474649/_build/results?buildId=1362636
Build error leg or test failing: Build Tests step in tvos-arm64 Release AllSubsets_CoreCLR_RuntimeTests and tvos-arm64 Release AllSubsets_Mono_RuntimeTests
Pull request: https://github.com/dotnet/runtime/pull/125441

Affected CI jobs: build_tvos_arm64_Release_AllSubsets_CoreCLR_RuntimeTests, build_tvos_arm64_Release_AllSubsets_Mono_RuntimeTests

## Error Message

Fill the error message using [step by step known issues guidance](https://github.com/dotnet/arcade/blob/main/Documentation/Projects/Build%20Analysis/KnownIssueJsonStepByStep.md).

```json
{
"ErrorMessage": "NuGet.RestoreEx.targets(19,5): error : Object reference not set to an instance of an object",
"ErrorPattern": "",
"BuildRetry": true,
"ExcludeConsoleLog": true
}
```

## Summary

NuGet restore fails with a NullReferenceException in `MSBuildStaticGraphRestore.GetDependencyGraphSpec` during RuntimeTests build on tvos legs:

```
.dotnet/sdk/11.0.100-preview.3.26170.106/NuGet.RestoreEx.targets(19,5): error : Object reference not set to an instance of an object.
at NuGet.Build.Tasks.Console.MSBuildStaticGraphRestore.b__37_3(RestoreProjectAdapter project)
at NuGet.Build.Tasks.Console.MSBuildStaticGraphRestore.<>c__DisplayClass38_0`1.b__0(KeyValuePair`2 project)
```

> [!NOTE]
> This issue was filed with the assistance of GitHub Copilot.

### Known issue validation
**Build: :mag_right:** https://dev.azure.com/dnceng-public/public/_build/results?buildId=1362636
**Error message validated:** `[NuGet.RestoreEx.targets(19,5): error : Object reference not set to an instance of an object`]
**Result validation:** :white_check_mark: Known issue matched with the provided build.
**Validation performed at:** 4/2/2026 10:17:33 AM UTC

### Report
#### Summary
|24-Hour Hit Count|7-Day Hit Count|1-Month Count|
|---|---|---|
|0|0|0|