CBL-4452: Migrate to nullable reference types in tests

[borrrden]

Massive amount of lines changed but 95% of it is just removing #nullable disabled and then addressing the various ? and ! needed. I tried to be mostly diligent about adding a null check to the test before adding ! but toward the end and the 1000th time doing it for basically the same functionality I gave up.

CBL-5547 This uncovered three nullability regressions though (you can see them in the non test files). I filed one as a ticket, but will cover all three with the same ticket.

I also added in an assembly property to not auto append the git SHA to the version, so that it remains pure as x.y.z-bdddd