(maint) Refactor shared example from parser/compiler_spec

Previously this spec's tests were a shared_example that had been shared
between runs with parser=current and parser=future.  The future compiler
tests were moved out into
spec/integration/parser/future_compiler_spec.rb in 7a6a7d8.  This maint
fix just removes the unused rspec construction of shared_examples_for
from around the specs in parser/compiler_spec, since they are only being
run against the current parser now.
(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
Merge pull request #2764 from jpartlow/issue/master/pup-2756-fix-undef-acceptance-for-future-parser

Issue/master/pup 2756 fix undef acceptance for future parser