Allow constructing wxGTK wxTextMeasure with NULL font.

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

include/wx/gtk/private/textmeasure.h

@@ -49,7 +49,6 @@ protected:
                                          wxArrayInt& widths,
                                          double scaleX);
 
-
     // This class is only used for DC text measuring with GTK+ 2 as GTK+ 3 uses
     // Cairo and not Pango for this. However it's still used even with GTK+ 3
     // for window text measuring, so the context and the layout are still