Fix division by zero error on performance page

For preview and staging environments, we often send no messages
in a single day. This is currently causing a `DivisionByZero` error
that is rendering the page with no results. This makes it impossible
to look at preview/staging and see if the performance page is
working correctly or not.

(psycopg2.errors.DivisionByZero) division by zero

[SQL: SELECT CAST(ft_processing_time.bst_date AS TEXT) AS date, ft_processing_time.messages_total AS ft_processing_time_messages_total, ft_processing_time.messages_within_10_secs AS ft_processing_time_messages_within_10_secs, (ft_processing_time.messages_within_10_secs / CAST(ft_processing_time.messages_total AS FLOAT)) * %(param_1)s AS percentage
FROM ft_processing_time
WHERE ft_processing_time.bst_date >= %(bst_date_1)s AND ft_processing_time.bst_date <= %(bst_date_2)s ORDER BY ft_processing_time.bst_date]
[parameters: {'param_1': 100, 'bst_date_1': datetime.date(2021, 11, 12), 'bst_date_2': datetime.date(2021, 11, 19)}]
(Background on this error at: http://sqlalche.me/e/14/9h9h)

I've fixed this by falling back to 100.0% for days we send
no messages. Maybe some argument that it should be N/A rather than
100% but I think it doesn't really matter as this is only
going to affect preview and staging as we will never have a day
sending no messages in production.

app/dao/fact_processing_time_dao.py

@@ -1,6 +1,7 @@
 from datetime import datetime
 
 from sqlalchemy.dialects.postgresql import insert
+from sqlalchemy.sql.expression import case
 
 from app import db
 from app.dao.dao_utils import autocommit
@@ -37,8 +38,13 @@ def get_processing_time_percentage_for_date_range(start_date, end_date):
         FactProcessingTime.bst_date.cast(db.Text).label("date"),
         FactProcessingTime.messages_total,
         FactProcessingTime.messages_within_10_secs,
-        ((FactProcessingTime.messages_within_10_secs / FactProcessingTime.messages_total.cast(
-            db.Float)) * 100).label("percentage")
+        case([
+            (
+                FactProcessingTime.messages_total > 0,
+                ((FactProcessingTime.messages_within_10_secs / FactProcessingTime.messages_total.cast(db.Float)) * 100)
+            ),
+            (FactProcessingTime.messages_total == 0, 100.0)
+        ]).label("percentage")
     ).filter(
         FactProcessingTime.bst_date >= start_date,
         FactProcessingTime.bst_date <= end_date