fix(instances): the grouped instance list without computes has a table body

The grouped list opens one tbody per compute, so with no compute the
sortable table had none, and sortable.js threw on page load ("Cannot read
properties of undefined (reading 'rows')"). An empty tbody is rendered
in that case.

instances/templates/allinstances_index_grouped.html

@@ -88,6 +88,8 @@
                 {% endfor %}
             {% endif %}
         </tbody>
+    {% empty %}
+        <tbody></tbody>{# sortable.js needs a table body #}
     {% endfor %}
 </table>
 

instances/tests/test_index.py

@@ -2,11 +2,13 @@
 
 from unittest.mock import PropertyMock, patch
 
+from appsettings.models import AppSettings
 from computes.models import Compute
 from django.contrib.auth import get_user_model
 from django.test import TestCase
 from django.urls import reverse
 from instances.models import Instance
+from lxml import html
 
 
 class InstancesIndexTests(TestCase):
@@ -42,3 +44,10 @@ class InstancesIndexTests(TestCase):
         self.assertEqual(response.status_code, 200)
         self.assertNotContains(response, "test4")
         self.assertEqual(list(response.context["computes"][0].instance_set.all()), [])
+
+    def test_the_grouped_list_without_computes_has_a_table_body(self):
+        # sortable.js reads the first tbody's rows: without one it throws and leaves the table unsortable
+        AppSettings.objects.filter(key="VIEW_INSTANCES_LIST_STYLE").update(value="grouped")
+        self.client.force_login(get_user_model().objects.create_superuser("grouped-admin", "g@example.com", "pw"))
+        page = html.fromstring(self.client.get(reverse("instances:index")).content)
+        self.assertEqual(len(page.xpath("//table[@data-sortable]/tbody")), 1)