Fix history storing in wxWebViewWebKit with custom schemes.

When the history has just been cleared there is no existing item so we should add the new history item even in this case.

Fixes #15446.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74768 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775