(maint) Fix getparam() spec failure on MRI 1.8

Without this patch applied we're getting the following spec failure, but
only in the MRI 1.8 matrix cells.

    Failures:

      1) getparam when compared against a resource with params
         Failure/Error: should run.with_params('User[dan]', '').and_return('')
         ArgumentError:
           interning empty string
         # ./vendor/ruby/1.8/gems/puppet-3.0.2/lib/puppet/parser/resource.rb:42:in `intern'
         # ./vendor/ruby/1.8/gems/puppet-3.0.2/lib/puppet/parser/resource.rb:42:in `[]'
         # ./lib/puppet/parser/functions/getparam.rb:29:in `real_function_getparam'
         # ./vendor/ruby/1.8/gems/puppet-3.0.2/lib/puppet/parser/functions.rb:63:in `send'
         # ./vendor/ruby/1.8/gems/puppet-3.0.2/lib/puppet/parser/functions.rb:63:in `function_getparam'
         # ./vendor/ruby/1.8/gems/rspec-puppet-0.1.5/lib/rspec-puppet/matchers/run.rb:8:in `call'
         # ./vendor/ruby/1.8/gems/rspec-puppet-0.1.5/lib/rspec-puppet/matchers/run.rb:8
         # ./vendor/ruby/1.8/gems/rspec-puppet-0.1.5/lib/rspec-puppet/matchers/run.rb:24:in `call'
         # ./vendor/ruby/1.8/gems/rspec-puppet-0.1.5/lib/rspec-puppet/matchers/run.rb:24
         # ./vendor/ruby/1.8/gems/rspec-expectations-2.11.3/lib/rspec/matchers/extensions/instance_eval_with_args.rb:11:in `instance_exec'
         # ./vendor/ruby/1.8/gems/rspec-expectations-2.11.3/lib/rspec/matchers/extensions/instance_eval_with_args.rb:11:in `instance_eval_with_args'
         # ./vendor/ruby/1.8/gems/rspec-expectations-2.11.3/lib/rspec/matchers/matcher.rb:60:in `matches?'
         # ./vendor/ruby/1.8/gems/rspec-expectations-2.11.3/lib/rspec/expectations/handler.rb:9:in `handle_matcher'
         # ./vendor/ruby/1.8/gems/rspec-core-2.11.1/lib/rspec/core/subject.rb:64:in `should'
         # ./spec/functions/getparam_spec.rb:29
         # ./vendor/ruby/1.8/gems/rspec-core-2.11.1/lib/rspec/core/example.rb:113:in `instance_eval'
         # ./vendor/ruby/1.8/gems/rspec-core-2.11.1/lib/rspec/core/example.rb:113:in `run'
         # ./vendor/ruby/1.8/gems/rspec-core-2.11.1/lib/rspec/core/example.rb:253:in `with_around_each_hooks'
         # ./vendor/ruby/1.8/gems/rspec-core-2.11.1/lib/rspec/core/example.rb:110:in `run'
         # ./vendor/ruby/1.8/gems/rspec-core-2.11.1/lib/rspec/core/example_group.rb:378:in `run_examples'
         # ./vendor/ruby/1.8/gems/rspec-core-2.11.1/lib/rspec/core/example_group.rb:374:in `map'
         # ./vendor/ruby/1.8/gems/rspec-core-2.11.1/lib/rspec/core/example_group.rb:374:in `run_examples'
         # ./vendor/ruby/1.8/gems/rspec-core-2.11.1/lib/rspec/core/example_group.rb:360:in `run'
         # ./vendor/ruby/1.8/gems/rspec-core-2.11.1/lib/rspec/core/example_group.rb:361:in `run'
         # ./vendor/ruby/1.8/gems/rspec-core-2.11.1/lib/rspec/core/example_group.rb:361:in `map'
         # ./vendor/ruby/1.8/gems/rspec-core-2.11.1/lib/rspec/core/example_group.rb:361:in `run'
         # ./vendor/ruby/1.8/gems/rspec-core-2.11.1/lib/rspec/core/command_line.rb:28:in `run'
         # ./vendor/ruby/1.8/gems/rspec-core-2.11.1/lib/rspec/core/command_line.rb:28:in `map'
         # ./vendor/ruby/1.8/gems/rspec-core-2.11.1/lib/rspec/core/command_line.rb:28:in `run'
         # ./vendor/ruby/1.8/gems/rspec-core-2.11.1/lib/rspec/core/reporter.rb:34:in `report'
         # ./vendor/ruby/1.8/gems/rspec-core-2.11.1/lib/rspec/core/command_line.rb:25:in `run'
         # ./vendor/ruby/1.8/gems/rspec-core-2.11.1/lib/rspec/core/runner.rb:69:in `run'
         # ./vendor/ruby/1.8/gems/rspec-core-2.11.1/lib/rspec/core/runner.rb:8:in `autorun'
         # ./vendor/ruby/1.8/bin/rspec:23

This patch addresses the problem by explicitly returning an empty string if the
string itself is empty.  This avoids trying to convert an empty string to a
symbol which is the root cause of the problem.