Handle cases where the parser returns nil

The Future Parser will return nil if a manifest file contains no statements
(I.E. it is empty or full of comments). Handle these cases and ensure the
`enumerate` method of the YARD parser always returns an Array.

lib/puppetx/yardoc/pops/yard_transformer.rb

@@ -45,5 +45,10 @@ module Puppetx::Yardoc::Pops
     def transform_Positioned(o)
       YARDStatement.new(o)
     end
+
+    # nil in... nil out!
+    def transform_NilClass(o)
+      nil
+    end
   end
 end

lib/puppetx/yardoc/yard/parser.rb

@@ -22,7 +22,10 @@ module Puppetx::Yardoc::YARD
     end
 
     def enumerator
-      @transformer.transform(@parse_result)
+      statements = @transformer.transform(@parse_result)
+
+      # Ensure an array is returned and prune any nil values.
+      Array(statements).compact
     end
 
   end