Only emit warnings for non-empty `<defs>` nodes.

Inkscape likes to put empty `<defs>` nodes in the file, which don't do
anything, so we technically support them.

svg/src/lib.rs

@@ -156,8 +156,10 @@ impl BuiltSVG {
                 self.result_flags.insert(BuildResultFlags::UNSUPPORTED_CLIP_PATH_NODE);
             }
             NodeKind::Defs { .. } => {
+                if node.has_children() {
                     self.result_flags.insert(BuildResultFlags::UNSUPPORTED_DEFS_NODE);
                 }
+            }
             NodeKind::Filter(..) => {
                 self.result_flags.insert(BuildResultFlags::UNSUPPORTED_FILTER_NODE);
             }