fix(similarity): add log for end of backfill and no cohort (#73134)

adds a log to the similarity backfill if there was no cohort provided

src/sentry/tasks/embeddings_grouping/backfill_seer_grouping_records_for_project.py

@@ -251,6 +251,10 @@ def call_next_backfill(
         # TODO: delete project redis key here if needed?
         # call the backfill on next project
         if not cohort:
+            logger.info(
+                "backfill finished, no cohort",
+                extra={"project_id": project_id},
+            )
             return
 
         if isinstance(cohort, str):