Handle NULL flags argument in wxOSX wxNotebook::HitTest().

Don't crash if flags are not specified, this parameter is optional.

Closes #15256.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74238 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775