add assert

Author: kkrik-es

server/src/main/java/org/elasticsearch/index/codec/tsdb/es819/ES819TSDBDocValuesProducer.java

@@ -390,6 +390,7 @@ public BlockLoader.Block tryRead(
                 int offset,
                 BlockDocValuesReader.ToDouble toDouble
             ) throws IOException {
+                assert toDouble == null;
                 if (ords instanceof BaseDenseNumericValues denseOrds) {
                     var block = tryReadAHead(factory, docs, offset);
                     if (block != null) {