Bug 54868 - isDisambig results in AttributeError

Set _pageprops to {} per default before updating from live site as
- a page without any pageprops has no corresponding entry in the pagedict and, thus, Page._pageprops will not be set in api.update_page()
- the pageprops may be removed after a forced update

Same problem (ValueError in Page) for Page.coordinates() if the page has no coordinates fixed in Page.

Change-Id: I557733ab88d43b4577678dfc0c36ae383bc7dce5
Merge "Bug 54868 - isDisambig results in AttributeError"