fix check es host

Author: WenDing-Y

Diff for: seatunnel-connectors-v2/connector-elasticsearch/src/main/java/org/apache/seatunnel/connectors/seatunnel/elasticsearch/source/ElasticsearchSourceSplitEnumerator.java

@@ -207,6 +207,9 @@ public void notifyCheckpointComplete(long checkpointId) throws Exception {}
 
     public List<IndexDocsCount> getIndexDocsCount(String index) {
         List<String> hosts = connConfig.get(ElasticsearchBaseOptions.HOSTS);
+        if (hosts.isEmpty()) {
+            throw new IllegalArgumentException("hosts must not be null nor empty");
+        }
         String endpoint =
                 String.format(
                         "%s/_cat/indices/%s?h=index,docsCount&format=json", hosts.get(0), index);