Fill size limit with group length when undefined

The size_limit can be evaluated as undefined because the attribute is set when
it is None on the server side.

issue11782
review424131003
(grafted from fed9fc5cfdfb69e30ab166b4a121c9a201e761fb)

--HG--
branch : 6.0

src/view/form.js

@@ -3242,7 +3242,7 @@ function eval_pyson(value){
                     size_limit = record.expr_eval(this.attributes.size);
                 }
                 if (this._readonly) {
-                    if (size_limit === null) {
+                    if ((size_limit === null) || (size_limit === undefined)) {
                         size_limit = this.screen.group.length;
                     } else {
                         size_limit = Math.min(