Fix empty table message

Bug was happening because:

```python
bool(list())
>>> False
```

```python
bool((item for item in list()))
>>> True
```

i.e. generator expressions cast to boolean are `True`, even if they’re
empty – Python doesn’t evaluate them.

This was causing the functional tests to fail because it was taking too
long for any table rows to appear on the page.

tests/app/main/views/test_activity.py

@@ -9,6 +9,7 @@ from bs4 import BeautifulSoup
 from app.main.views.jobs import get_time_left, get_status_filters
 from tests import notification_json
 from tests.conftest import SERVICE_ONE_ID
+from tests.app.test_utils import normalize_spaces
 from freezegun import freeze_time
 
 
@@ -135,6 +136,23 @@ def test_can_show_notifications(
     assert json_content.keys() == {'counts', 'notifications'}
 
 
+def test_shows_message_when_no_notifications(
+    client_request,
+    mock_get_detailed_service,
+    mock_get_notifications_with_no_notifications,
+):
+
+    page = client_request.get(
+        'main.view_notifications',
+        service_id=SERVICE_ONE_ID,
+        message_type='sms',
+    )
+
+    assert normalize_spaces(page.select('tbody tr')[0].text) == (
+        'No messages found'
+    )
+
+
 @pytest.mark.parametrize((
     'initial_query_arguments,'
     'form_post_data,'