Fix assert in webview sample when viewing pages without a title.

Fix assert in webview sample when viewing pages without a title.

Trying to append a menu item with empty label resulted in an assert (at least
under wxGTK but probably elsewhere too), and in any case wasn't very useful,
so use "(untitled)" instead in this case.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72174 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775