In strict mode do not allow undefined variables to return empty string

[chamons]

The number of times I meant $(Foo) vs @(Foo) and burned 10 minutes debugging suggests this is a common mistake.

[JustinSchneiderPBI]

Love it! This has been a problem for us with large build refactors that change the props import sequence, like migration to SDK-style projects and Central Package Management. We have to just spot-check some and check the conceptual props import sequence makes conceptual sense. One potential impediment in our build, I found that pre-restore evaluations can run into this too easily if a props file is loaded that depends on an expected nuget package's props. Hope this is related; apologies if I'm missing some subtlety or better solutions here.

[stan-sz]

Marking as an idea for #1777