test for no placeholder

When you create a Page, you have to save it before you get to do anything with placeholders. Once you've saved it for the first time, an empty placeholder is created.

When using placeholders in a model outside the CMS, you can create a new one in the admin and start editing it, but if you have a placeholder field in it, it will say:

> You must save the page first to add plugins.

That's good, but it will also throw up some JavaScript errors. For each placeholder, you'll get a ``SyntaxError: Unexpected token ')'``, because ``cms/templates/admin/cms/page/widgets/plugin_editor.html`` tries to populate some JavaScript using a missing ``placeholder`` variable.

It doesn't stop anything from working, but it's unsightly.

Simply adding ``{% if placeholder %}`` around the JavaScript solves the problem.