Allow empty ItemsToSign when building VisualFSharp with core msbuild

[ellahathaway]

Needed for dotnet/sdk#44855

VisualFSharp.sln only produces artifacts files when built with full framework MSBuild, so we need to allow empty sign list when building VisualFSharp.sln with Core MSBuild in order to pass -sign.

[github-actions]

✅ No release notes required

[ViktorHofer]

Let's wait with merging this until we figured out why artifacts don't get created.

[psfinaki]

Blocking for the above reasons to prevent accidental merge.

[ellahathaway]

Let's wait with merging this until we figured out why artifacts don't get created.

I think the lack of artifacts is expected here. VisualFSharp.sln only produces artifacts on full msbuild. Alternatively, it looks like Microsoft.FSharp.Compiler.sln is producing the correct artifacts (albeit not all of them are shipping/aligned with linux). See dotnet/sdk#44855 (comment)

[ellahathaway]

I'm closing this PR in favor of opening an issue against fsharp. In the case where we're building on windows, -noVisualStudio is not passed, -sign is passed, and we're building with .NET Core MSBuild, the build fails. The correct thing to do here would be to avoid passing -sign to the build in that case.