Skip elements with false conditions during evaluation

[dibarbet]

Issue was that a specific item group had a condition to prevent an empty property. Inside the item group, it used [MSBuild]::VersionEquals which cannot handle an empty value.

However, evaluation was attempting to evaluate it anyway - which threw an error causing the project to fail to load.

This changes evaluation to skip item groups with false conditions.

DRAFT - confirming with msbuild if this property should always be set on evaluation

Resolves dotnet/vscode-csharp#6356

[jasonmalinowski]

I guess I can sign off on this one, in the sense that people smarter than I would know the real questions to ask.