[web] Represent CSS identity transforms as 'none' instead of null (flutter#18288)

Author: jason-simmons

lib/web_ui/lib/src/engine/util.dart

@@ -82,7 +82,7 @@ String float64ListToCssTransform(Float32List matrix) {
     return float64ListToCssTransform3d(matrix);
   } else {
     assert(transformKind == TransformKind.identity);
-    return null;
+    return 'none';
   }
 }