testsuite: Add test for empty glshader node

Everyone should draw the error pink here, because that's what the
renderers not supporting it do, and it's also what the default shader
does.

So no matter if a renderer supports GL shaders or not, it should draw
the same pink.

testsuite/gsk/compare/empty-glshader.node

@@ -0,0 +1 @@
+glshader {}

testsuite/gsk/meson.build

@@ -64,6 +64,7 @@ compare_render_tests = [
   'empty-container',
   'empty-cross-fade',
   'empty-debug',
+  'empty-glshader',
   'empty-inset-shadow',
   'empty-linear-gradient',
   'empty-mask',