Don't add empty classes

XML nodes were being added for empty class lists when they didn't need to be. This was causing issues for some parsers when trying to parse them empty class list. This commit simply ommits a package if it has no applicable files

lib/simplecov-cobertura.rb

@@ -49,6 +49,7 @@ def result_to_xml(result)
         end
 
         groups.map do |name, files|
+          next if files.empty?
           packages.add_element(package = REXML::Element.new('package'))
           set_package_attributes(package, name, files)