publisher: correct v2 page updates with unset metadata

Correcting v2 API implementation recently introduced that incorrectly
prepared metadata objects when Confluence does not report any metadata
information for a page (which in turn results in `AttributeError`
exceptions).

Signed-off-by: James Knight <james.d.knight@live.com>

sphinxcontrib/confluencebuilder/publisher.py

@@ -1518,8 +1518,8 @@ def _update_page(self, page, page_name, data, parent_id=None):
         pending_new_labels = []
         pending_prop_requests = []
         if self.api_mode == 'v2':
-            orig_metadata = page.get('metadata', None)
-            update_metadata = update_page.pop('metadata', None)
+            orig_metadata = page.get('metadata', {})
+            update_metadata = update_page.pop('metadata', {})
 
             # configure parent page for this page update
             #