Data cleanup script: exclude previously-cleaned

The cleanup of old deleted comments and topics was repeatedly hitting
the same items even though they had already been cleaned up on previous
runs. This just explicitly excludes all the ones that have already had
their user_id info removed, so it will only hit ones that actually need
it.

tildes/scripts/clean_private_data.py

@@ -89,7 +89,9 @@ class DataCleaner():
         updated = (
             self.db_session.query(Comment)
             .filter(
-                Comment.deleted_time <= self.retention_cutoff)  # type: ignore
+                Comment.deleted_time <= self.retention_cutoff,  # type: ignore
+                Comment.user_id != 0,
+            )
             .update({
                 'user_id': 0,
                 'markdown': '',
@@ -108,7 +110,9 @@ class DataCleaner():
         updated = (
             self.db_session.query(Topic)
             .filter(
-                Topic.deleted_time <= self.retention_cutoff)  # type: ignore
+                Topic.deleted_time <= self.retention_cutoff,  # type: ignore
+                Topic.user_id != 0,
+            )
             .update({
                 'user_id': 0,
                 'title': '',