The font is explicitly documented as being possibly NULL in the base class and
wxMSW handles NULL font just fine, so also handle it in the GTK version.
See #14706.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72844 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775