Post build event is run an extra time with empty values when multi-targeting

### Issue Description

I've added a post build event to my csproj to sign DLLs (getting this to run in the Azure DevOps pipeline was hell -- took a day.)

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The `Condition` is required beause the target is run for ever `

Start by reproducing the issue with the attached ConsoleAppPostBuildEvent.zip and its csproj, using TargetFrameworks with the AzureSignTool post-build target. Trace the multi-target build to find why the target runs with empty values; done means it runs once per framework and not for an additional empty target.

Written by the indexing model from the issue text.