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.

app/main/views/jobs.py

@@ -386,7 +386,7 @@ def get_job_partials(job):
 
 
 def add_preview_of_content_to_notifications(notifications):
-    return (
+    return [
         dict(
             preview_of_content=(
                 str(Template(notification['template'], notification['personalisation']))
@@ -396,4 +396,4 @@ def add_preview_of_content_to_notifications(notifications):
             **notification
         )
         for notification in notifications
-    )
+    ]