Ensure a customRender that returns null falls back on the default behaviour

Author: erickok

lib/html_parser.dart

@@ -257,8 +257,8 @@ class HtmlParser extends StatelessWidget {
         tree.attributes,
         tree.element,
       );
-      assert(render is InlineSpan || render is Widget);
       if (render != null) {
+        assert(render is InlineSpan || render is Widget);
         return render is InlineSpan
             ? render
             : WidgetSpan(