Validation of vars was not properly checking for forward references to
string variables. This, coupled with a null vs undefined bug in
subsequent use, meant that forward reference variables were ending up
with a literal string value of 'undefined'.
This also fixes the test for visual-keyboard-compiler, where the fixture
was actually buggy and was the trigger for investigating this problem.
Fixes: #12403
Relates-to: #12395
44e28e21a6