Fixed bug where empty placeholder would render as 'None'

This bug would appear when "placeholder" template tag appeared in a
template being rendered by a non-CMS view.

The render_page_placeholder function had a bare "return" instead of
returning an explicit empty string.

See also the code that would be executed for the same case in
django-cms 3.3.x (in cms_tags.py:render_tag in that release):

        page = request.current_page
        if not page or page == 'dummy':
            if nodelist:
                return nodelist.render(context)
            return ''

cms/plugin_rendering.py

@@ -254,7 +254,7 @@ def render_page_placeholder(self, slot, context, inherit, nodelist=None):
         current_page = self.current_page
 
         if not current_page:
-            return
+            return ''
 
         content = self._render_page_placeholder(
             context=context,