Variants were not being properly handled in the `flag-resolver`: The
fact that the default value of the variant is not falsy made it so we
never asked the external flag resolver for the value.
This also moves the logic from `Variant | undefined` to `Variant` where
we use the `getDefaultVariant()` helper method to return us a [default
variant](55274e4953/src/variant.ts (L37-L42)).