Do not use instanceof on value of Date and DateTime of Dict field

Instead we must use the attributes isDate and isDateTime.

Closes #13845
(grafted from c5b215bb328104a51f8d26d1f3e9e50f588d6568)

--HG--
branch : 7.0

sao/src/view/form.js

@@ -5382,8 +5382,7 @@ function eval_pyson(value){
             return this._parse(this.format, this.input.val());
         },
         set_value: function(value) {
-            if ((value instanceof Sao.DateTime) ||
-                (value instanceof Sao.Date)) {
+            if (value && (value.isDate || value.isDateTime)) {
                 value = this._format(this.format, value);
             } else {
                 value = '';