(PUP-2898) Fix errors on ruby 1.8.7

Ruby 1.8.7 doesn't allow trailing commas in parameter lists. This
problem was present in the resource_expressions_spec file and is removed
by this commit.

Ruby 1.8.7 doesn't have an empty? method for Symbol. This changes the
evaluator_impl to compare against the EMPTY_STRING constant instead,
which does work on 1.8 and 1.9.