Fixes problems in WindowsDesktop SDK due to _TargetFrameworkVersionWithoutV being undefined sometimes (#1713)

* Add _TargetFrameworkVersionValue

* Define _UndefinedTargetFrameworkVersion

* Update conditions using _TargetFrameworkVersionValue & _UndefinedTargetFrameworkVersion

* Format conditions

* Add a default value for _TargetFrameworkVersionValue in .props file, and fixup the default value in .targets file.

Fixup an incorrect comment

* Add _UndefinedTargetFrameworkVersion and _TargetFrameworkVersionValue to Pbt.targets (used for dotnet/wpf builds only)