[opt](test)Add expected indices count check (apache#39024)

Only when the expected_indices_count is 0, the tablet may not have the
index file.

regression-test/plugins/plugin_curl_requester.groovy

@@ -255,7 +255,8 @@ logger.info("Added 'be_report_task' function to Suite")
 Suite.metaClass.check_nested_index_file = { ip, port, tablet_id, expected_rowsets_count, expected_indices_count, format -> 
     def (code, out, err) = http_client("GET", String.format("http://%s:%s/api/show_nested_index_file?tablet_id=%s", ip, port, tablet_id))
     logger.info("Run show_nested_index_file_on_tablet: code=" + code + ", out=" + out + ", err=" + err)
-    if (code == 500) {
+    // only when the expected_indices_count is 0, the tablet may not have the index file.
+    if (code == 500 && expected_indices_count == 0) {
         assertEquals("E-6003", parseJson(out.trim()).status)
         assertTrue(parseJson(out.trim()).msg.contains("not found"))
         return