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
Instant[] timestamps = new Instant[] {
                time,
                time.plusSeconds(1),
                time.plusSeconds(5),
                time.minus(30, ChronoUnit.DAYS),
                time.plus(30, ChronoUnit.DAYS) };
            for (Instant timestamp : timestamps) {
                var indexRequest = new Request("POST", "/k8s/_doc");
                indexRequest.setJsonEntity(DOC.replace("$time", formatInstant(timestamp)));
                var response = client().performRequest(indexRequest);
                assertOK(response);
                var responseBody = entityAsMap(response);
                assertThat((String) responseBody.get("_index"), backingIndexEqualTo("k8s", 2));
            }
        }
    }
    public void testUpdateComponentTemplateDoesNotFailIndexTemplateValidation() throws IOException {
        var request = new Request("POST", "/_component_template/custom_template");
        request.setJsonEntity("""
            {
                "template": {
                    "settings": {
                        "index": {
                            "number_of_replicas": 1
                        }
                    }
                }
            }
            """);
        client().performRequest(request);