Use default dict for default job stats

This is just some refactoring.

`defaultdict` is a data structure which won’t raise a `KeyError` if you
try to access a key that doesn’t exist.

By passing `int` as the first argument, trying to access the value of
any key that doesn’t exists will return the value of `int()`, ie `0`

app/notify_client/job_api_client.py

@@ -1,3 +1,4 @@
+from collections import defaultdict
 
 from notifications_python_client.base import BaseAPIClient
 from app.notify_client import _attach_current_user
@@ -16,12 +17,7 @@ class JobApiClient(BaseAPIClient):
 
     @staticmethod
     def __convert_statistics(job):
-        results = {
-            'sending': 0,
-            'delivered': 0,
-            'failed': 0,
-            'requested': 0
-        }
+        results = defaultdict(int)
         if 'statistics' in job:
             for outcome in job['statistics']:
                 if outcome['status'] in ['failed', 'technical-failure', 'temporary-failure', 'permanent-failure']: