Apparently, DirectWrite fonts can be created only from TrueType fonts and
therefore only such fonts can be used with Direct2D-based wxGraphicsContext.
When unsuported GDI font is passed to CreateFont() then no graphics font is
created and this unsuccessful attempt is signalled by returning
wxNullGraphicsFont. This null object can be used in e.g. wxGC::SetFont()
to check if font was actually created.
See #17790.