A composable index template with no `template` defined in the body is mistakingly always assumed to not be a time series template. Even if it refers to a component template that has the `index.mode` setting set to `time_series` and the component template defines mappings with dimension fields or routing paths.

Incorrectly this causes to create a data stream with a tsdb index *without* time series start and end time settings set. The fact that this can happen is another bug and is being tracked #98833.

What should happen in this case is that a tsdb data stream is created with a valid tsdb backing index (with correctly generated time series start and end time settings).

This bug was found as part investigating #98792 and is the cause of that bug.
modules/data-streams/src/javaRestTest/java/org/elasticsearch/datastreams/TsdbDataStreamRestIT.java
var response = client().performRequest(indexRequest);
            assertOK(response);
        }
        {
            var putComposableIndexTemplateRequest = new Request("POST", "/_index_template/1");
            putComposableIndexTemplateRequest.setJsonEntity(NON_TSDB_TEMPLATE);
            client().performRequest(putComposableIndexTemplateRequest);
        }
        {
            {
                
                var getDataStreamsRequest = new Request("GET", "/_data_stream");
                var getDataStreamResponse = client().performRequest(getDataStreamsRequest);
                assertOK(getDataStreamResponse);
                var dataStreams = entityAsMap(getDataStreamResponse);
                assertThat(ObjectPath.evaluate(dataStreams, "data_streams.0.name"), equalTo("k8s"));
                assertThat(ObjectPath.evaluate(dataStreams, "data_streams.0.indices"), hasSize(1));
                assertThat(ObjectPath.evaluate(dataStreams, "data_streams.0.time_series"), notNullValue());
            }
            var rolloverRequest = new Request("POST", "/k8s/_rollover");
            var rolloverResponse = client().performRequest(rolloverRequest);
            assertOK(rolloverResponse);
            var rolloverResponseBody = entityAsMap(rolloverResponse);
            assertThat(rolloverResponseBody.get("rolled_over"), is(true));
            {
                
                var getDataStreamsRequest = new Request("GET", "/_data_stream");
                var getDataStreamResponse = client().performRequest(getDataStreamsRequest);
                assertOK(getDataStreamResponse);
                var dataStreams = entityAsMap(getDataStreamResponse);