Property functions with a single `_` as an argument return bad results

[rainersigwald]

<Project>
    <Target Name="Go">
      <Warning Text="Surprise! $([System.String]::Join('_', 'asdf', 'jkl'))" />
    </Target>
</Project>

New:

❯ artifacts\bin\bootstrap\net472\MSBuild\Current\Bin\MSBuild.exe .\foo.proj -tl:false
MSBuild version 17.12.0-dev-24373-01+9f69926a2 for .NET Framework

S:\msbuild\foo.proj(18,7): warning : Surprise!

Expected:

❯ msbuild .\foo.proj -tl:false
MSBuild version 17.12.0-preview-24367-07+486dbb4a9 for .NET Framework


S:\msbuild\foo.proj(18,7): warning : Surprise! asdf_jkl

This is the root cause of errors in dotnet/sdk#42258 because of https://github.com/dotnet/wpf/blob/d21766ccfcdeb05aa3feeb6a195c01dbf2e1a5fb/src/Microsoft.DotNet.Wpf/src/PresentationBuildTasks/Microsoft.WinFX.targets#L376 and was introduced by 9f69926, specifically

msbuild/src/Build/Evaluation/Expander.cs

Line 3597 in 8b03742

if (args.Any(a => "_".Equals(a)))