gstsink: Do not advertise dmabufs with no formats

We return an empty format list when dmabufs aren't supported, not NULL.

And the sink was treating the empty format list by setting no fourccs
on the caps, which GStreamer conveniently interpreted as "any",
not as "none".

modules/media/gtkgstsink.c

@@ -219,7 +219,7 @@ gtk_gst_sink_get_caps (GstBaseSink *bsink,
     {
       GdkDmabufFormats *formats = gdk_display_get_dmabuf_formats (self->gdk_display);
 
-      if (formats)
+      if (formats && gdk_dmabuf_formats_get_n_formats (formats) > 0)
         {
           tmp = gst_caps_from_string (DMABUF_TEXTURE_CAPS);
           add_drm_formats_and_modifiers (tmp, formats);