Fix props for the time referred from T4.

When a project refereed T4 with Roslyn workspace API, $OutputPath
and $TargetFramework property become empty. It causes formatting
$DocumentationFile UNC like path and causes T4 error.
This commit add workaround for it.

Author: yfakariya

MsgPack.Common.props

@@ -39,7 +39,10 @@
   <PropertyGroup Condition="'$(TargetFramework)' == 'MonoAndroid10' or '$(TargetFramework)' == 'Xamarin.iOS10'">
     <SignAssembly>false</SignAssembly>
   </PropertyGroup>
-  <PropertyGroup>
+  <PropertyGroup Condition="'$(OutputPath)' == '' and '$(TargetFramework)' == ''">
+    <DocumentationFile>$(AssemblyName).XML</DocumentationFile>
+  </PropertyGroup>
+  <PropertyGroup Condition="'$(OutputPath)' != '' or '$(TargetFramework)' != ''">
     <DocumentationFile>$(OutputPath)\$(TargetFramework)\$(AssemblyName).XML</DocumentationFile>
   </PropertyGroup>
   <PropertyGroup Condition="'$(TargetFramework)' == 'uap10.0'">