Add ProcessFrameworkReferences target to Empty.targets

First of all, I hate this whole filtering model. It's super hacky and constantly causes issues. dotnet/source-build#4047 tracks this.

Here we need to also empty the `ProcessFrameworkReferences` target as it still runs as it has a Before/AfterTargets hook. Keep it running is causing issues when RuntimeIdentifier is set to a custom value even though the project should be excluded.

Unblocks dotnet/sdk#45362

src/Microsoft.DotNet.Arcade.Sdk/tools/Empty.targets

@@ -22,5 +22,6 @@
   <Target Name="Test"/>
   <Target Name="Pack"/>
   <Target Name="Publish"/>
+  <Target Name="ProcessFrameworkReferences" />
 
 </Project>