[MVC] Fix MSB4186 error (#31066)

Fix MSB4186 error if a project references Microsoft.AspNetCore.Mvc.Testing that is not itself an ASP.NET project, such as a helper library in a test suite.

Author: martincostello

src/Mvc/Mvc.Testing/src/Microsoft.AspNetCore.Mvc.Testing.targets

@@ -29,7 +29,7 @@
     </Target>
 
   <Target Name="_AddContentRootForProjectReferences" BeforeTargets="PrepareResources" DependsOnTargets="_ResolveMvcTestProjectReferences">
-    <ItemGroup>
+    <ItemGroup Condition="'@(_ContentRootProjectReferences->Count())' != 0">
       <_ManifestProjects Include="%(_ContentRootProjectReferences.FusionName)">
         <ContentRoot>$([System.IO.Path]::GetDirectoryName(%(_ContentRootProjectReferences.MSBuildSourceProjectFile)))</ContentRoot>
       </_ManifestProjects>