(PUP-2756) Fix test of future parser evaluation of empty string

An empty string is no longer undef when parser is future, and will
evaluate as true in a conditional test. This commit fixes a test that
was failing this check for the future parser. This, and several like
it, was an acceptance test which was just compiling a simple manifest
with some conditional logic to test evaluation of conditionals. Since
acceptance tests are expensive, I've pulled them back into the
integration specs.

The actual test being fixed is
acceptance/tests/apply/conditionals/should_evaluate_empty.rb which is
now the two 'evaluates empty string as <false|true>' specs in the new
spec/integration/parser/conditionals_spec.rb