do not bookkeep incremental aggregate if inc dumps do not exist

Author: amCap1712

listenbrainz_spark/stats/incremental/incremental_stats_engine.py

@@ -230,4 +230,5 @@ def run(self) -> Iterator[Dict]:
         self.prepare_final_aggregate()
         results = self.generate_stats()
         yield from self.create_messages(results, self.only_inc, self.message_creator)
-        self.bookkeep_incremental_aggregate()
+        if incremental_listens_exist():
+            self.bookkeep_incremental_aggregate()