[FIX] website_event: unblock button when no reserve

In 11.0, this change e9454e79 solved the use case of:

- opening the registration of a ticket
- discard

=> the page must be reloaded to register a ticket

A new report is that since 9.0, if we try to register 0 ticket we would
also have to reload the page.

This commit backports e9454e79 and solves the 0 ticket registration.

opw-1851622
closes #24966

addons/website_event/static/src/js/website_event.js

@@ -13,16 +13,23 @@ $(document).ready(function () {
             ev.preventDefault();
             ev.stopPropagation();
             var $form = $(ev.currentTarget).closest('form');
+            var $button = $(this);
             var post = {};
             $("select").each(function() {
                 post[$(this)[0].name] = $(this).val();
             });
             ajax.jsonRpc($form.attr('action'), 'call', post).then(function (modal) {
+                // Only needed for 9.0 up to saas-14
+                if (modal === false) {
+                    $button.prop('disabled', false);
+                    return;
+                }
                 var $modal = $(modal);
                 $modal.find('.modal-body > div').removeClass('container'); // retrocompatibility - REMOVE ME in master / saas-19
-                $modal.after($form).modal();
+                $modal.insertAfter($form).modal();
                 $modal.on('click', '.js_goto_event', function () {
                     $modal.modal('hide');
+                    $button.prop('disabled', false);
                 });
             });
         });