Good point, I think unfortunately it’s not possible at this stage to filter on Not defined keyword. Please consider creating a feature request at dev.piwik.org or sponsoring such feature if needed
Good news: you can now use our new Search Engine Keywords Performance plugin which lets you import your Google, Bing and Yahoo keywords into your Piwik. Learn more about the reports provided and how to import keywords on the plugin page and in the Search Keywords Monitor User Guide.