rocksdb: fix out of bounds when flush is empty (#3605)

Author: arnetheduck

execution_chain/db/core_db/backend/rocksdb_desc.nim

@@ -72,12 +72,13 @@ proc commit*(
     # Write to disk if everyone that opened a session also committed it
     ?rdb.db.write(session.batch)
 
-    # This flush forces memtables to be written to disk, which is necessary given
-    # the use of vector memtables which have very bad lookup performance.
-    rdb.db.flush(session.families.mapIt(it.handle())).isOkOr:
-      # Not sure what to do here - the commit above worked so it would be strange
-      # to have an error here
-      warn "Could not flush database", error
+    if session.families.len > 0:
+      # This flush forces memtables to be written to disk, which is necessary given
+      # the use of vector memtables which have very bad lookup performance.
+      rdb.db.flush(session.families.mapIt(it.handle())).isOkOr:
+        # Not sure what to do here - the commit above worked so it would be strange
+        # to have an error here
+        warn "Could not flush database", error
 
   ok()