[FIX] survey: fix empty date in survey could not be submitted

Purpose
=======

A date field not marked as "mandatory" could not be left blank in the survey.

That was caused by the form returning "Invalid date" instead of an
empty value when submitting.

closes odoo/odoo#30788

addons/survey/static/src/js/survey.js

@@ -134,7 +134,7 @@ if(!the_form.length) {
             var date_fields = $form.find('div.date > input.form-control');
             for (var i=0; i < date_fields.length; i++) {
                 var el = date_fields[i];
-                var moment_date = moment(el.value);
+                var moment_date = el.value !== '' ? moment(el.value) : '';
                 if (moment_date) {
                     moment_date.toJSON = function () {
                         return this.clone().locale('en').format('YYYY-MM-DD');