Add two unit-tests for skipped targets with inputs but no outputs

[KirillOsenkov]

Building this project results in logging a TaskParameterEventArgs with kind == SkippedTargetInputs, where itemType == null and Items == null:

<Project>

  <ItemGroup>
    <_CodesignItems Include="foo" />
  </ItemGroup>

  <Target Name="Build"
          Inputs="@(_CodesignItems)"
          Outputs="@(_CodesignItems -> '%(CodesignStampFile)')">
  </Target>

</Project>

This other case results in a friendlier message:

<Project>

  <Target Name="Build"
          Inputs="@(_CodesignItems)"
          Outputs="@(_CodesignItems -> '%(CodesignStampFile)')">
  </Target>

</Project>

The TaskParameterEventArgs is logged here:

msbuild/src/Build/BackEnd/Components/RequestBuilder/TargetUpToDateChecker.cs

Line 357 in de1d7a2

itemType: null,

The friendlier message is logged here:

msbuild/src/Build/BackEnd/Components/RequestBuilder/TargetUpToDateChecker.cs

Lines 483 to 484 in de1d7a2

	_loggingService.LogComment(_buildEventContext, MessageImportance.Low,
	"SkipTargetBecauseNoOutputsDetail");

We should add a couple of tests for the above scenarios and perhaps avoid logging both TaskParameterEventArgs if the item array is empty.

[KirillOsenkov]

Originally reported in the viewer: KirillOsenkov/MSBuildStructuredLog#561

[KirillOsenkov]

I have fixed the viewer to at least not crash with a NullReferenceException in that case.