utils/viewer-cairo.c: fix empty-body

Fix the following build failure raised
since version 1.50.5 and
https://gitlab.gnome.org/GNOME/pango/-/commit/cd08fb7402498e6ea542b4628447547477ac212e:

../utils/viewer-cairo.c: In function 'cairo_vector_view_create':
../utils/viewer-cairo.c:228:5: error: suggest braces around empty body in an 'if' statement [-Werror=empty-body]
     ;
     ^

Fixes:
 - http://autobuild.buildroot.org/results/dee3d631474f83b345f22eb26c59a305c32258f8



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>