Full solution build fails within Visual Studio

@ViktorHofer It seems like you've broken this in https://github.com/dotnet/dotnet/pull/2177/ with the `` inside the PBT project:

https://github.com/dotnet/wpf/blob/4425f57bdd5513d42b20e54dcc0e3a6071797922/src/Microsoft.DotNet.Wpf/src/PresentationBuildTasks/PresentationBuildTasks.csproj#L3-L4

Wouldn't something like a conditional on whether `NetFrameworkToolCurrent` doesn't have a value do the same job with less unpredictable behaviour since I assume when you're building on unix this one will be unset?

Without removing the empty TargetFramework this fails within Visual Studio with `error MSB4057: The target "GetTargetPath" does not exist in the project.`