We do not post when no data

make more explicit that if the entire history of a metric is null then
we do not post a record to the latest data dataset.

tests/transformers/fixtures/transactions_explorer_example_data.json

@@ -72,7 +72,7 @@
         "digital_takeup": 0.965537995968002,
         "end_at": "2012-04-01T00:00:00+00:00",
         "number_of_digital_transactions": 2184914,
-        "number_of_transactions": 2262898,
+        "number_of_transactions": null,
         "period": "year",
         "service_id": "sorn-innit",
         "total_cost": 11767069.6,
@@ -92,7 +92,7 @@
         "digital_takeup": 0.9756123825537215,
         "end_at": "2013-01-01T00:00:00+00:00",
         "number_of_digital_transactions": 2301214,
-        "number_of_transactions": 2358738,
+        "number_of_transactions": null,
         "period": "year",
         "service_id": "sorn-innit",
         "total_cost": 6203480.9399999995,

tests/transformers/tasks/test_latest_transaction_explorer_values.py

@@ -66,16 +66,6 @@
         "dashboard_slug": "sorn",
         "type": "seasonally-adjusted"
     },
-    {
-        "_id": encode_id('sorn', 'number_of_transactions'),
-        "_timestamp": "2013-04-01T00:00:00+00:00",
-        "end_at": "2012-04-01T00:00:00+00:00",
-        "number_of_transactions": 2262898,
-        "period": "year",
-        "service_id": "sorn-innit",
-        "dashboard_slug": "sorn",
-        "type": "seasonally-adjusted"
-    },
     {
         "_id": encode_id('sorn', 'total_cost'),
         "_timestamp": "2013-04-01T00:00:00+00:00",
@@ -182,7 +172,7 @@ def test_compute(self, mock_dashboard_finder, mock_dataset):
             'data-group': 'transactions-explorer',
             'data-type': 'spreadsheet'}})
 
-        assert_that(len(transformed_data), is_(12))
+        assert_that(len(transformed_data), is_(11))
         assert_that(transformed_data, contains_inanyorder(*data_to_post))
 
     @patch("performanceplatform.client.DataSet.from_group_and_type")