Closes #747.

What's wrong

Running dotnet outdated against a solution that references no projects (e.g. one an AI agent scaffolded but hasn't populated yet) crashes with an unhandled System.IO.FileNotFoundException instead of a clean message. Per the reporter:

Unhandled exception. System.IO.FileNotFoundException: Could not find file 'C:\Users\...\AppData\Local\Temp\2zbjhlwk.elv'.
...
   at DotNetOutdated.Core.Services.DependencyGraphService.GenerateDependencyGraphAsync(...)
   at DotNetOutdated.Core.Services.ProjectAnalysisService.AnalyzeProjectAsync(...)

Root cause

When the target solution/project has zero restorable projects, dotnet msbuild ... /t:Restore,GenerateRestoreGraphFile exits successfully but never writes the RestoreGraphOutputPath file (nothing to restore). DependencyGraphService unconditionally read that file on success, so the missing-file read escaped as a raw, unhandled I/O exception.

What changed

DependencyGraphService.GenerateDependencyGraphAsync now checks whether the restore graph file actually exists before reading it, and throws the same CommandValidationException the method already uses for the restore-failure branch — which the CLI already catches and reports cleanly — with a new friendly message ("No projects were found to analyze in '{path}'. Make sure the solution references at least one project.").

Testing

• Added test-projects/empty-solution/ (a real .sln with zero project references) + an end-to-end test that runs the CLI against it and asserts a clean exit instead of a crash.
• Added a unit test in DependencyGraphServiceTests that mocks the MSBuild-succeeds-but-no-output-file case directly.
• Full suite: 209 tests, 208 passed, 1 pre-existing unrelated skip, 0 failed.
• Manually reproduced the exact crash from the issue on the pre-fix build, and confirmed the fix replaces it with a friendly message (exit code 1, no stack trace).