Assert instead of ! for nullable

[rainersigwald]

Minor nitpick about #11537.

[rainersigwald]

Test failure from Build_WithCustomBuildArgs_ShouldEmitWarningOnFramework is

The provided expression
    should have single item but had
1
    items and was
[Microsoft.Build.Framework.BuildWarningEventArgs (64925097)]

Which . . . what? Retrying.