Setting the primary document to None doesn't work

In the Page Settings dialog you cannot set the primary document to None after you have choosen a document before.

Reproduction steps:

• create a page prototype that has a container on the page
• create a page using this prototype
• add a component in the container that contains a link to a document, for example the Banner component
• use the page settings dialog to set the primary document: the document you choose in the previous step show up as a suggestion
• save and publish the channel manager changes
• edit the same page again and use the page settings dialog: set the primary document to "None" and save

Result: the primary document is not removed and the hst:relativecontentpath property on the sitemap item node remains. (Although the CM thinks you made a change that you can publish).