Silent NPE fix in persistent process (#9909)

Just avoid NPE in codes.

oap-server/server-storage-plugin/storage-elasticsearch-plugin/src/main/java/org/apache/skywalking/oap/server/storage/plugin/elasticsearch/base/BatchProcessEsDAO.java

@@ -86,7 +86,9 @@ public class BatchProcessEsDAO extends EsDAO implements IBatchDAO {
 
     @Override
     public void endOfFlush() {
-        // Flush forcedly due to this kind of metrics has been pushed into the bulk processor.
-        bulkProcessor.flush();
+        // Flush forcibly due to this kind of metrics has been pushed into the bulk processor.
+        if (bulkProcessor != null) {
+            bulkProcessor.flush();
+        }
     }
 }