Fix #719: fix crash in metrics when no changes to approve

kinto-remote-settings/src/kinto_remote_settings/signer/updater.py

@@ -310,14 +310,14 @@ def get_source_records(self, **kwargs):
     def get_destination_records(self, **kwargs):
         return self._get_records(self.destination, **kwargs)
 
-    def push_records_to_destination(self, request):
+    def push_records_to_destination(self, request) -> int:
         dest_records, _dest_timestamp = self.get_destination_records()
         source_records, _source_timestamp = self.get_source_records()
         new_records = records_diff(source_records, dest_records)
         changes_count = len(new_records)
 
         if len(new_records) == 0:
-            return None
+            return 0
 
         # Update the destination collection.
         for record in new_records:

kinto-remote-settings/tests/signer/test_events.py

@@ -498,6 +498,21 @@ def test_review_approved_is_triggered(self):
         assert isinstance(self.events[-1], signer_events.ReviewApproved)
         assert self.events[-1].changes_count == 2
 
+    def test_changes_count_is_zero_when_no_changes(self):
+        self.app.delete(
+            self.source_collection + "/records",
+            headers=self.headers,
+        )
+
+        self.app.patch_json(
+            self.source_collection,
+            {"data": {"status": "to-sign"}},
+            headers=self.headers,
+        )
+
+        assert isinstance(self.events[-1], signer_events.ReviewApproved)
+        assert self.events[-1].changes_count == 0
+
     def test_review_approved_is_not_triggered_on_resign(self):
         self.app.patch_json(
             self.source_collection,