Avoid error when TargetFrameworkProfile is \r (#3208)
Avoid error when TargetFrameworkProfile is \r (#3208)

Project System telemetry indicates a significant number of failures that
report

```
The expression "[System.IO.Path]::Combine(D:\Temp\, .NETFramework,Version=v4.5,Profile=
 .AssemblyAttributes.cs)" cannot be evaluated. Illegal characters in path. C:\Program Files (x86)\Microsoft Visual Studio\2017\Professional\MSBuild\15.0\Bin\Microsoft.Common.CurrentVersion.targets
```

This seems to be because the XML entity for a carriage return gets
embedded in the file. We don't currently know what would cause that.

This is a tactical fix: instead of comparing against '', explicitly
use `String.IsNullOrWhitespace` which knows about all kinds of
whitespace, even bare \r.

Closes #3197.
mono/msbuild,AndyGerlicher/msbuild,Microsoft/msbuild,sean-gilliam/msbuild,jeffkl/msbuild,mono/msbuild,sean-gilliam/msbuild,jeffkl/msbuild,jeffkl/msbuild,rainersigwald/msbuild,mono/msbuild,sean-gilliam/msbuild,jeffkl/msbuild,AndyGerlicher/msbuild,AndyGerlicher/msbuild,chipitsine/msbuild,Microsoft/msbuild,chipitsine/msbuild,mono/msbuild,cdmihai/msbuild,Microsoft/msbuild,rainersigwald/msbuild,rainersigwald/msbuild,sean-gilliam/msbuild,Microsoft/msbuild,cdmihai/msbuild,cdmihai/msbuild,cdmihai/msbuild,AndyGerlicher/msbuild,chipitsine/msbuild,chipitsine/msbuild,rainersigwald/msbuild