SI-8330: Mismatch in stack heights

The SI-8233 / 9506d52 missed one case when we need to DROP a null from
a stack: when unboxed Unit is an expected type. If we forgot to do that
in a context where two branches were involved we could end up with
unbalanced stack sizes.

Let's fix that omission and a test covering that specific case to the
original test for SI-8233.

Fixes SI-8330.