[FIX] web: domain field should never be considered unset

Indeed, false was already considered as "[]" in edit mode.

addons/web/static/src/js/fields/basic_fields.js

@@ -2027,6 +2027,15 @@ var FieldDomain = AbstractField.extend({
     // Public
     //--------------------------------------------------------------------------
 
+    /**
+     * A domain field is always set since the false value is considered to be
+     * equal to "[]" (match all records).
+     *
+     * @override
+     */
+    isSet: function () {
+        return true;
+    },
     /**
      * @override isValid from AbstractField.isValid
      * Parsing the char value is not enough for this field. It is considered