Don't merge ri files if is nil/empty

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@10267 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/rdoc/ri/ri_descriptions.rb

@@ -95,8 +95,10 @@ module RI
       if @comment.nil? || @comment.empty?
         @comment = old.comment
       else
-        @comment << SM::Flow::RULE.new
-        @comment.concat old.comment
+        unless old.comment.nil? or old.comment.empty? then
+          @comment << SM::Flow::RULE.new
+          @comment.concat old.comment
+        end
       end
     end