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.
server/src/test/java/org/elasticsearch/cluster/metadata/MetadataTests.java
Exception e = expectThrows(IllegalArgumentException.class, () -> mdBuilder.put("logs-postgres", "logs-postgres-eu", null, null));
        assertThat(e.getMessage(), equalTo("alias [logs-postgres] refers to a non existing data stream [logs-postgres-eu]"));
        mdBuilder.put(DataStreamTestHelper.randomInstance("logs-postgres-eu"));
        mdBuilder.put("logs-postgres", "logs-postgres-eu", null, null);
        Metadata metadata = mdBuilder.build();
        assertThat(metadata.dataStreamAliases().get("logs-postgres"), notNullValue());
        assertThat(metadata.dataStreamAliases().get("logs-postgres").getDataStreams(), containsInAnyOrder("logs-postgres-eu"));
    }
    public void testDeleteDataStreamShouldUpdateAlias() {
        Metadata.Builder mdBuilder = Metadata.builder();
        mdBuilder.put(DataStreamTestHelper.randomInstance("logs-postgres-eu"));
        mdBuilder.put("logs-postgres", "logs-postgres-eu", null, null);
        mdBuilder.put(DataStreamTestHelper.randomInstance("logs-postgres-us"));
        mdBuilder.put("logs-postgres", "logs-postgres-us", null, null);
        mdBuilder.put(DataStreamTestHelper.randomInstance("logs-postgres-au"));
        mdBuilder.put("logs-postgres", "logs-postgres-au", null, null);
        Metadata metadata = mdBuilder.build();
        assertThat(metadata.dataStreamAliases().get("logs-postgres"), notNullValue());
        assertThat(
            metadata.dataStreamAliases().get("logs-postgres").getDataStreams(),
            containsInAnyOrder("logs-postgres-eu", "logs-postgres-us", "logs-postgres-au")
        );
        mdBuilder = Metadata.builder(metadata);
        mdBuilder.removeDataStream("logs-postgres-us");
        metadata = mdBuilder.build();
        assertThat(metadata.dataStreamAliases().get("logs-postgres"), notNullValue());
        assertThat(
            metadata.dataStreamAliases().get("logs-postgres").getDataStreams(),
            containsInAnyOrder("logs-postgres-eu", "logs-postgres-au")
        );